December 19, 2018

Tran Nguyen
Chief Operating Officer and Financial Officer
Prothena Corporation plc
Adelphi Plaza
Upper George's Street
D n Laoghaire
Co. Dublin, A96 T927, Ireland

       Re: Prothena Corp plc
           Form 10-K for the Fiscal Year Ended December 31, 2017
           Filed February 26, 2018
           Form 10-Q for the Quarterly Period Ended September 30, 2018
           Filed November 6, 2018
           File No. 001-35676

Dear Mr. Nguyen:

        We have limited our review of your filings to the financial statements
and related
disclosures and have the following comment. In our comment, we may ask you to
provide us
with information so we may better understand your disclosure.

       Please respond to this comment within ten business days by providing the
requested
information or advise us as soon as possible when you will respond. If you do
not believe our
comment applies to your facts and circumstances, please tell us why in your
response.

       After reviewing your response to the comment, we may have additional
comments.

Form 10-Q for the Quarterly Period Ended September 30, 2018

Notes to the Condensed Consolidated Financial Statements
8. Significant Agreements
Celgene Collaboration Agreement, page 18

1.     Please provide the following information so we may further evaluate your
accounting
       treatment for the Celgene Collaboration Agreement.
         You state that at inception of the collaboration, you did not have
material, distinct
           performance obligations, as you were not obligated to transfer US or
Global licenses.
           However, Article 7 of the Master Collaboration Agreement states that
"Prothena
 Tran Nguyen
FirstName LastNameTran Nguyen
Prothena Corporation plc
Comapany19, 2018
December NameProthena Corporation plc
December 19, 2018 Page 2
Page 2
FirstName LastName
             hereby grants to Celgene a non-exclusive, worldwide, fully-paid
up, royalty-free right
             and license with the right to grant sublicenses under the Prothena
IP for Celgene to
             conduct its activities and perform its obligations under this
Agreement," which appears
             to have occurred on March 20, 2018. In addition, Article 7
indicates that you granted a
             research and development license to Celgene. Explain this apparent
inconsistency and
             how these two licenses will be utilized by Celgene in its own
research activities.
             Explain the factors that you considered in concluding that the
following activities did
             not constitute material performance obligations at inception: the
two licenses granted
             by you to Celgene, your responsibilities to participate on the
joint steering committee,
             your responsibilities to conduct research activities at its
direction and your
             responsibilities to prosecute, maintain and enforce Prothena
Collaboration Patents and
             Program Patents.
             You state that the Company is responsible for all research and
development activity
             through completion of Phase 1 clinical studies. However, you also
state that the
             Company is not obligated to perform development activities under
the development
             plan during preclinical and Phase 1 clinical trials. Explain this
apparent inconsistency.
             Explain how the discovery, preclinical and clinical development
activities to be
             performed by you "at the Company discretion" will be coordinated
with joint steering
             committee decisions, related Celgene research activities and
Celgene's acquisition of
             viable collaboration programs through exercise of its US Rights.
             You state that the initial transaction price of $110.2 million
will be deferred until
             "Celgene exercises the US Right and receives control of the US
license." Furthermore,
             you state that the total transaction price will be allocated to
each of the Company's
             performance obligations "at the point that Celgene receives the
license for each
             program." Provide a more detailed explanation of the triggering
events and expected
             timing for this revenue recognition process, including any
activity measures that you
             expect to utilize. In this regard, tell us your estimate of the
number of collaboration
             programs, for which you expect to complete Phase 1 clinical trials
over the six year-
             term of the Agreement that in turn would be acquired by Celgene
through exercise of
             its US Rights.


       In closing, we remind you that the company and its management are
responsible for the
accuracy and adequacy of their disclosures, notwithstanding any review,
comments, action or
absence of action by the staff.
 Tran Nguyen
Prothena Corporation plc
December 19, 2018
Page 3

      You may contact Tabatha McCullom at 202-551-3658 or Frank Wyman at
202-551-
3660 with any questions.



FirstName LastNameTran Nguyen                        Sincerely,
Comapany NameProthena Corporation plc
                                                     Division of Corporation
Finance
December 19, 2018 Page 3                             Office of Healthcare &
Insurance
FirstName LastName