S-8

As filed with the Securities and Exchange Commission on August 10, 2022

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-8

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

PROTHENA CORPORATION PUBLIC LIMITED COMPANY

(Exact Name of Registrant as Specified in its Charter)

Ireland (State or Other Jurisdiction of Incorporation or Organization) 98-1111119 (I.R.S. Employer Identification No.)

77 Sir John Rogerson’s Quay, Block C
Grand Canal Docklands
Dublin 2,	D02 VK60,	Ireland
(Address of principal executive offices, including Zip Code)

Prothena Corporation plc 2018 Long Term Incentive Plan

Prothena Corporation plc 2020 Employment Inducement Incentive Plan

(Full Title of the Plans)

Michael J. Malecek

Chief Legal Officer and Company Secretary

Prothena Corporation plc

c/o Prothena Biosciences Inc.

331 Oyster Point Boulevard

South San Francisco, California 94080

(650) 837-8550

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copy to:

Jennifer Fitchen

Sidley Austin LLP

1001 Page Mill Road, Building 1

Palo Alto, California 94304

(650) 565-7000

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act:

Large accelerated filer	☒	Accelerated filer	☐
Non-accelerated filer	☐	Smaller reporting company	☐
		Emerging growth company	☐
If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.			☐

PART IINFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

The information called for in Part I of Form S-8 is not being filed with or included in this Form S-8 (by incorporation by reference or otherwise) in accordance with the rules and regulations of the Securities and Exchange Commission (the “Commission”). This Registration Statement on Form S-8 is filed by Prothena Corporation plc (the “Registrant”) regarding the Prothena Corporation plc 2018 Long Term Incentive Plan, as amended (the “2018 Plan”), and the Prothena Corporation plc 2020 Employment Inducement Incentive Plan, as amended (together with the 2018 Plan, the “Plans”). Documents containing the information required by Part I of the Registration Statement will be sent or given to participants in the Plans as specified by Rule 428(b)(1) under the Securities Act of 1933, as amended (the “Securities Act”).

PART IIINFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.

The information incorporated by reference herein is considered to be part of this Registration Statement, and later information filed with the Commission will update and supersede this information. The following documents filed by the Registrant with the Commission are incorporated herein by reference:

(a) The Registrant’s Registration Statement on Form 10 (File No. 001-35676) filed with the Commission under Section 12(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), on October 1, 2012, including the description of the Registrant’s ordinary shares contained therein, and any amendment or report filed for the purpose of updating such description;

(b) The Registrant’s Annual Report on Form 10-K for the year ended December 31, 2021, filed with the Commission on February 25, 2022;

(c) The Registrant’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2022, filed with the Commission on May 5, 2022;

(d) The Registrant’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2022, filed with the Commission on August 8, 2022; and

(e) The Registrant’s Current Reports on Form 8-K filed with the Commission on March 28, 2022, April 26, 2022 and May 20, 2022.

All documents that the Registrant subsequently files pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the filing of a post-effective amendment to the Registration Statement which indicates that all of the ordinary shares offered have been sold or which deregisters all of such shares then remaining unsold shall be deemed to be incorporated by reference in this Registration Statement and to be a part hereof from the date of the filing of such documents, except as to any portion of any future annual or quarterly report to shareholders or document or current report furnished under Items 2.02 or 7.01 of Form 8-K, and exhibits furnished on such form that relate to such items, that is not deemed filed under such provisions. For the purposes of this Registration Statement, any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this Registration Statement.

Under no circumstances will any information filed under current Items 2.02 or 7.01 of Form 8-K be deemed incorporated herein by reference unless such Form 8-K expressly provides to the contrary.

Item 4. Description of Securities.

Not applicable.

Item 5. Interests of Named Experts and Counsel.

Not applicable.

Item 6. Indemnification of Directors and Officers.

Under Irish law, a company may not exempt or indemnify its directors from liability for negligence or a breach of duty. However, where a breach of duty has been established, directors may be statutorily exempted by an Irish court from personal liability for negligence or breach of duty if, among other things, the court determines that they have acted honestly and reasonably, and that they may fairly be excused as a result.

The Irish Companies Act 2014 (the “Companies Act”) permits a company to pay the costs or discharge the liability of a director or the secretary only where judgment is given in his/her favor in any civil or criminal action in respect of such costs or liability, or where an Irish court grants relief because the director or secretary acted honestly and reasonably and ought fairly to be excused. This restriction does not apply to executives who are not directors or the secretary. Any obligation of an Irish company that purports to indemnify a director or secretary of an Irish company over and above this will be void under Irish law, whether contained in its articles of association or any contract between the director or secretary and the company.

The Registrant’s Amended and Restated Memorandum and Articles of Association (the “Constitution”), which has been publicly filed with the Commission, provides, so far as is permitted under the Companies Act, that every director and the secretary shall be entitled to be indemnified by the Registrant against all costs, charges, losses, expenses and liabilities incurred by him or her in the execution and discharge of his or her duties or in relation thereto including any liability incurred by him or her in defending any proceedings, civil or criminal, which relate to anything done or omitted or alleged to have been done or omitted by him or her as an officer or employee of the Registrant and in which judgement is given in his or her favour (or the proceedings are otherwise disposed of without any finding or admission of any material breach of duty on his or her part) or in which he or she is acquitted or in connection with any application under any statute for relief from liability in respect of any such act or omission in which relief is granted to him or her by the court.

In addition, the Constitution provides that the Registrant shall indemnify any of its current or former executives (excluding any directors or secretary), or any person who is serving or has served at the Registrant’s request as a director, executive or trustee of another company, joint venture, trust or other enterprise (each, a “Covered Person”), against expenses, including attorney’s fees, judgements, fines and amounts paid in settlement actually and reasonably incurred by him or her in connection with any threatened, pending, or completed action, suit or proceeding, whether civil, criminal, administrative or investigative, to which he or she was, is, or is threatened to be made a party, by reason of the fact that he or she is or was a Covered Person.

The directors and officers of the Registrant are covered by insurance policies indemnifying against certain liabilities, including certain liabilities arising under the Securities Act, which might be incurred by them in such capacities and against which they may not be indemnified by us.

The Registrant has also entered into indemnification agreements with each of its directors and executive officers. These agreements, among other things, require the Registrant to indemnify an indemnitee to the fullest extent permitted by applicable law, including indemnification of expenses such as attorneys’ fees, judgments, fines and settlement amounts incurred by the indemnitee in any action or proceeding, arising out of the person’s services as a director or executive officer.

Item 7. Exemption From Registration Claimed.

Not applicable.

Item 8. Exhibits.

Exhibit No.	Description
4.1	Amended and Restated Memorandum and Articles of Association (Constitution) of Prothena Corporation plc.(1)
4.2	Prothena Corporation plc 2018 Long Term Incentive Plan.(2)
4.3	First Amendment to the Prothena Corporation plc 2018 Long Term Incentive Plan.(3)
4.4	Second Amendment to the Prothena Corporation plc 2018 Long Term Incentive Plan.(4)
4.5	Third Amendment to the Prothena Corporation plc 2018 Long Term Incentive Plan. (5)
4.6	Prothena Corporation plc 2020 Employment Inducement Incentive Plan.(6)
4.7	First through Tenth Amendments to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan.(7)
4.8*	Eleventh Amendment to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan.
4.9*	Twelfth Amendment to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan.
4.10*	Thirteenth Amendment to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan.
5.1*	Opinion of A&L Goodbody.
23.1*	Consent of KPMG LLP.
23.3*	Consent of A&L Goodbody (included in Exhibit 5.1).
24.1*	Power of Attorney (included on the signature page hereto).
107*	Calculation of Filing Fee Table

(1) Incorporated by reference to Exhibit 3.1 of the Registrant’s Current Report on Form 8-K filed with the Commission on May 25, 2016.

(2) Incorporated by reference to Exhibit 10.1 of the Registrant’s Current Report on Form 8-K filed with the Commission on May 18, 2018.

(3) Incorporated by reference to Exhibit 10.1 of the Registrant’s Current Report on Form 8-K filed with the Commission on May 22, 2020.

(4) Incorporated by reference to Exhibit 10.1 of the Registrant’s Current Report on Form 8-K filed with the Commission on May 21, 2021.

(5) Incorporated by reference to Exhibit 10.1 of the Registrant’s Current Report on Form 8-K filed with the Commission on May 20, 2022.

(6) Incorporated by reference to Exhibit 10.2 of the Registrant’s Quarterly Report on Form 10-Q filed with the Commission on May 6, 2020.

(7)  Incorporated by reference to Exhibit 10.20 of the Registrant’s Annual Report on Form 10-K filed with the Commission on February 25, 2022.

* Filed herewith.

Item 9. Undertakings.

a. The undersigned Registrant hereby undertakes:

1. To file, during any period in which offers or sales are being made, a post-effective amendment to this Registration Statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities Act;

(ii) To reflect in the prospectus any facts or events arising after the effective date of this Registration Statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in this Registration Statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percent change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii) To include any material information with respect to the plan of distribution not previously disclosed in this Registration Statement or any material change to such information in this Registration Statement;

Provided, however, that: paragraphs (i) and (ii) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the Registrant pursuant to Sections 13 or 15(d) of the Exchange Act that are incorporated by reference in this Registration Statement.

2. That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

3. To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

b. The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant’s annual report pursuant to Sections 13(a) or 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in this Registration Statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

h. Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on this 10thday of August, 2022.

Prothena Corporation plc

/s/ Gene G. Kinney

Gene G. Kinney

President and Chief Executive Officer

POWER OF ATTORNEY

KNOW ALL PERSONS BY THESE PRESENT, that each person whose signature appears below does hereby constitute and appoint Gene G. Kinney, Tran B. Nguyen and Michael J. Malecek, and each of them, with full power of substitution and full power to act without the other, his true and lawful attorney-in-fact and agent to act for him in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement, and to file this Registration Statement, with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite and necessary to be done in order to effectuate the same as fully, to all intents and purposes, as they or he might or could do in person, hereby ratifying and confirming all that said attorneys-in-fact and agents, or any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

Name	Title	Date
/s/ Gene G. Kinney	President and Chief Executive Officer (Principal Executive Officer) and Director	August 10, 2022
Gene G. Kinney, Ph.D.
/s/ Tran B. Nguyen	Chief Financial Officer and Chief Strategy Officer (Principal Financial Officer)	August 10, 2022
Tran B. Nguyen
/s/ Karin L. Walker	Chief Accounting Officer (Principal Accounting Officer)	August 10, 2022
Karin L. Walker
/s/ Lars G. Ekman	Chairman of the Board	August 10, 2022
Lars G. Ekman, M.D., Ph.D.
/s/ Paula K. Cobb	Director	August 10, 2022
Paula K. Cobb
/s/ Richard T. Collier	Director	August 10, 2022
Richard T. Collier
/s/ Shane M. Cooke	Director	August 10, 2022
Shane M. Cooke
/s/ K. Anders O. Härfstrand	Director	August 10, 2022
K. Anders O. Härfstrand, M.D., Ph.D.
/s/ Oleg Nodelman	Director	August 10, 2022
Oleg Nodelman
/s/ Sanjiv K. Patel	Director	August 10, 2022
Sanjiv K. Patel, MBBS
/s/ Dennis J. Selkoe	Director	August 10, 2022
Dennis J. Selkoe, M.D.

EX-FILING FEES

Exhibit 107

Calculation of Filing Fee Table

Form S-8

(Form Type)

Prothena Corporation Public Limited Company

(Exact Name of Registrant as Specified in its Charter)

Table 1: Newly Registered Securities

Security type	Security Class Title	Fee Calculation Rule	Amount Registered (1)	Proposed Maximum Offering Price Per Unit			Maximum Aggregate Offering Price		Fee Rate	Amount of Registration Fee
Equity (2)	Ordinary Shares, par value $0.01 per share	Rule 457(c) and 457(h)	2,000,000	$	31.61	(3)	$	63,220,000	$0.0000927	$	5,860.49
Equity (4)	Ordinary Shares, par value $0.01 per share	Rule 457(c) and 457(h)	250,000	$	31.61	(3)	$	7,902,500	$0.0000927	$	732.56
Total Offering Amounts										$	6,593.06
Total Fee Offsets										$	—
Net Fee Due										$	6,593.06

(1) Pursuant to Rule 416 promulgated under the Securities Act of 1933, as amended (the “Securities Act”), this Registration Statement shall also cover any additional ordinary shares of Prothena Corporation plc (the “Registrant”) that become issuable under the Prothena Corporation plc 2018 Long Term Incentive Plan, as amended (the “2018 Plan”), and the Prothena Corporation plc 2020 Employment Inducement Incentive Plan, as amended (the “2020 Plan”), by reason of any share dividend, share split, recapitalization or other similar transaction effected without receipt of consideration that increases the number of outstanding shares of the Registrant’s ordinary shares.

(2) Represents an additional 2,000,000 ordinary shares available for future grants under the 2018 Plan pursuant to the Third Amendment to the Prothena Corporation plc 2018 Long Term Incentive Plan, which was approved by the Registrant’s shareholders on May 17, 2022.

(3) Estimated solely for the purpose of calculating the registration fee pursuant to Rules 457(c) and 457(h) of the Securities Act for the ordinary shares registered hereunder (based on the average of the high and low prices for the Registrant’s ordinary shares reported by The Nasdaq Stock Market on August 5 2022).

(4) Represents 250,000 ordinary shares reserved for issuance pursuant to equity awards granted under the 2020 Plan.

EX-4.8

Exhibit 4.8

ELEVENTH AMENDMENT TO THE

PROTHENA CORPORATION PLC

2020 EMPLOYMENT INDUCEMENT INCENTIVE PLAN

This Eleventh Amendment (this “Eleventh Amendment”) to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan (“2020 EIIP”), was made and adopted by the Board of Directors (“Board”) of Prothena Corporation plc, a public limited company organized under the laws of Ireland (the “Company”), on October 1, 2021 (the “Amendment Date”).

RECITALS

WHEREAS, the Company maintains the 2020 EIIP; and

WHEREAS, the Board believes it is in the best interests of the Company and its shareholders to amend the 2020 EIIP to increase the number of ordinary shares authorized for issuance under the 2020 EIIP.

NOW, THEREFORE, BE IT RESOLVED, that the 2020 EIIP is hereby amended as follows, effective as of the Amendment Date:

AMENDMENT

1.Section 2.28 of the 2020 EIIP is hereby amended and restated in its entirety as follows:

“2.28 “Overall Share Limit” means 1,385,000 Shares.”

2.This Eleventh Amendment shall be and hereby is incorporated into and forms a part of the 2020 EIIP, and except as expressly provided herein, all terms and conditions of the 2020 EIIP shall remain in full force and effect.

EX-4.9

Exhibit 4.9

TWELFTH AMENDMENT TO THE

PROTHENA CORPORATION PLC

2020 EMPLOYMENT INDUCEMENT INCENTIVE PLAN

This Twelfth Amendment (this “Twelfth Amendment”) to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan (“2020 EIIP”), was made and adopted by the Board of Directors (“Board”) of Prothena Corporation plc, a public limited company organized under the laws of Ireland (the “Company”), on July 1, 2022 (the “Amendment Date”).

RECITALS

WHEREAS, the Company maintains the 2020 EIIP; and

WHEREAS, the Board believes it is in the best interests of the Company and its shareholders to amend the 2020 EIIP to increase the number of ordinary shares authorized for issuance under the 2020 EIIP.

NOW, THEREFORE, BE IT RESOLVED, that the 2020 EIIP is hereby amended as follows, effective as of the Amendment Date:

AMENDMENT

1.Section 2.28 of the 2020 EIIP is hereby amended and restated in its entirety as follows:

“2.28 “Overall Share Limit” means 1,435,000 Shares.”

2.This Twelfth Amendment shall be and hereby is incorporated into and forms a part of the 2020 EIIP, and except as expressly provided herein, all terms and conditions of the 2020 EIIP shall remain in full force and effect.

EX-4.10

Exhibit 4.10

THIRTEENTH AMENDMENT TO THE

PROTHENA CORPORATION PLC

2020 EMPLOYMENT INDUCEMENT INCENTIVE PLAN

This Thirteenth Amendment (this “Thirteenth Amendment”) to the Prothena Corporation plc 2020 Employment Inducement Incentive Plan (“2020 EIIP”), was made and adopted by the Board of Directors (“Board”) of Prothena Corporation plc, a public limited company organized under the laws of Ireland (the “Company”), on August 1, 2022 (the “Amendment Date”).

RECITALS

WHEREAS, the Company maintains the 2020 EIIP; and

WHEREAS, the Board believes it is in the best interests of the Company and its shareholders to amend the 2020 EIIP to increase the number of ordinary shares authorized for issuance under the 2020 EIIP.

NOW, THEREFORE, BE IT RESOLVED, that the 2020 EIIP is hereby amended as follows, effective as of the Amendment Date:

AMENDMENT

1.Section 2.28 of the 2020 EIIP is hereby amended and restated in its entirety as follows:

“2.28 “Overall Share Limit” means 1,470,000 Shares.”

2.This Thirteenth Amendment shall be and hereby is incorporated into and forms a part of the 2020 EIIP, and except as expressly provided herein, all terms and conditions of the 2020 EIIP shall remain in full force and effect.

EX-5.1

Exhibit 5.1

Date	10 August 2022
Our Ref	AFC/ DEG 01405528

Prothena Corporation plc

77 Sir John Rogersons Quay

Block C Grand Canal Docklands

Dublin 2

D02 VK60

Ireland

Prothena Corporation plc (Prothena)

Dear Sirs

We are acting as Irish counsel for Prothena, a public limited company incorporated under the laws of Ireland, in connection with the proposed registration by Prothena of (i) 2,000,000 ordinary shares, par value $0.01 per share issuable under the Prothena Corporation plc 2018 Long Term Incentive Plan (as amended) (the 2018 LTIP) and (ii) 250,000 ordinary shares, par value $0.01 per share currently issuable under the Prothena Corporation plc 2020 Employment Inducement Incentive Plan (as amended) (the 2020 EIIP,together with the 2018 LTIP, the Plans andthe ordinary shares referred to in (i) and (ii) above, together the Ordinary Shares), pursuant to a Registration Statement on Form S-8 (the Registration Statement)to be filed by Prothena under the Securities Act of 1933, as amended. This Opinion is solely related to the Registration Statement.

In connection with this Opinion, we have reviewed copies of such corporate records of Prothena as we have deemed necessary as a basis for the opinions hereinafter expressed. In rendering this Opinion, we have examined, and have assumed the truth and accuracy of the contents of, such documents and certificates of officers of Prothena and of public officials as to factual matters and have conducted such searches in public registries in Ireland as we have deemed necessary or appropriate for the purposes of this Opinion but have made no independent investigation regarding such factual matters. In our examination we have assumed the truth and accuracy of the information contained in such documents, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified or photostatic copies and the authenticity of the originals of such documents.

We have further assumed that none of the resolutions and authorities of the shareholders and Directors of Prothena upon which we have relied have been varied, amended or revoked in any

CE Gill • JG Grennan • PD White • VJ Power • LA Kennedy • SM Doggett • B McDermott • S O’Riordan • M Sherlock • KP Allen • C Rogers • G O’Toole • JN Kelly • N O’Sullivan

MJ Ward • AC Burke • D Widger • C Christle • S Ó Cróinin • JW Yarr • DR Baxter • A McCarthy • JF Whelan • JB Somerville • MF Barr • AM Curran • A Roberts • RM Moore • D Main

J Cahir • M Traynor • PM Murray • P Walker • K Furlong • PT Fahy • D Inverarity • M Coghlan • DR Francis • A Casey • B Hosty • M O’Brien • L Mulleady • K Ryan • E Hurley • D Dagostino

R Grey • R Lyons • J Sheehy • C Carroll • SE Carson • P Diggin • J Williams • A O’Beirne • J Dallas • SM Lynch • M McElhinney • C Owens • AD Ion • K O'Connor • JH Milne

T Casey • M Doyle • CJ Comerford • R Marron • K O'Shaughnessy • S O'Connor • SE Murphy • D Nangle • A Lawler • C Ó Conluain • N McMahon • HP Brandt • A Sheridan • LM Byrne

N Cole • M Devane • D Fitzgerald • G McDonald • N Meehan • R O'Driscoll • B O'Malley • C Bollard • M Daly • D Geraghty • LC Kennedy • E Mulhern • E O'Keeffe

Consultants: Professor JCW Wylie • MA Greene • AV Fanagan • PM Law • C Duffy • SW Haughey • PV Maher

respect or have expired and that the Ordinary Shares will be issued in accordance with such resolutions and authorities.

We further assume that Prothena will comply with all covenants given and representations and warranties made in connection with the Plans and that Prothena will receive consideration equal to the aggregate of the par value and any premium required to be paid up on the Ordinary Shares issued pursuant to awards under the Plans and that such consideration will be in cash and/or otherwise provided in accordance with Irish law.

Having made such further investigation and reviewed such other documents as we have considered requisite or desirable, subject to the foregoing and to the within additional qualifications and assumptions, we are of the opinion that:

1.Prothena has been duly incorporated and is an existing public limited company under the laws of Ireland; and

2.the Ordinary Shares have been duly authorised and when issued in accordance with the terms of the 2018 LTIP and the 2020 EIIP (as applicable) and the options or other equity awards granted or to be granted thereunder, will be validly issued, fully paid and not subject to calls for any additional payments (“nonassessable”).

.In rendering this Opinion, we have confined ourselves to matters of Irish law. We express no opinion on any laws other than the laws of Ireland (and the interpretation thereof) in force as at the date hereof. This Opinion speaks only as of its date.

We hereby consent to the filing of this Opinion with the Securities Exchange Commission as an exhibit to the Registration Statement.

This Opinion is being delivered to you and may not be relied upon or distributed to any other person without our prior written consent.

The Opinion is governed by and construed in accordance with the laws of Ireland.

Yours faithfully

/s/ A&L Goodbody LLP

A&L Goodbody LLP

M-55917664-2

2

EX-23.1

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the use of our reports dated February 25, 2022, with respect to the consolidated financial statements of Prothena Corporation plc, and the effectiveness of internal control over financial reporting, incorporated herein by reference.

/s/ KPMG LLP

San Francisco, California

August 8, 2022