April 3, 2019

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 reviewed your January 22, 2019 response as supplemented by your
February 28,
2019 response to our comment letter and have the following comments. In our
comments, we
may ask you to provide us with information so we may better understand your
disclosure.

       Please respond to the comments 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
comments apply to your facts and circumstances, please tell us why in your
response.

       After reviewing your response to the comments, we may have additional
comments. Unless we note otherwise, our references to prior comments are to
comments in
our December 19, 2018 letter.

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

Item 1. Financial Statements (unaudited)
Notes to the Condensed Consolidated Financial Statements
8. Significant Agreements
Celgene Collaboration Agreement, page 18

1.     Please propose revised disclosure that:
        articulates consistently when Celgene would have "decision making
authority over
 Tran Nguyen
FirstName LastNameTran Nguyen
Prothena Corporation plc
Comapany NameProthena Corporation plc
April 3, 2019
April 2 2019 Page 2
Page 3,
FirstName LastName
             development activities and all regulatory, manufacturing and
commercialization
             activities in the U.S." We note that the first paragraph of the
Overview section
             indicates this occurs upon Celgene's exercise of the option to
acquire U.S. Rights
             whereas the first paragraph of the Celgene U.S. and Global Rights
and Licenses
             section indicates this occurs "following Phase 1."
             clarifies whether the exclusive license to develop and
commercialize that Celgene will
             receive upon exercise of its options for U.S. Rights and Global
Rights includes the
             right to manufacture.
             quantifies the regulatory and commercial milestones per program
the Company is
             eligible to receive after Celgene's exercise of U.S. Rights.
2.       Please analyze for us whether your contract with Celgene includes one
or more implied
         promises by Prothena to perform research and development services
prior to the filing of
         an IND. We make this request because your response appears to be
limited to whether an
         explicit promise exists, nonetheless promises may be implied by an
entity's customary
         business practices, published policies or specific statements (ASC
606-10-25-16); implied
         promises need not be enforceable at law (ASC No. 2014-09 BC87); and
the illustrative
         statements below appear to imply such a promise.
           "And that's essentially the way we kind of move that into the R&D.
So again to
             reiterate, we're responsible for research and development through
Phase I. So to be
             clear, that includes CMC, that includes regulatory, basically
every other function in
             there."   March 20, 2018 Prothena Corporation PLC Neuroscience R&D
             Collaboration Investor Call
           "Prothena will lead and have final decision-making authority over
all programs
             through Phase I, unless Celgene elects otherwise at its expense."
- March 20, 2018
             Prothena Corporation PLC Neuroscience R&D Collaboration Investor
Call
           "And as you may recall, Prothena is responsible for the execution of
these programs
             through the preclinical and early clinical stages, dependent upon
various clinical
             option exercise [indiscernible] by Celgene."  Q4 2018 Earnings
Transcript
           "... this collaboration provides Prothena the opportunity to work
with a premier
             scientific partner and the resources and flexibility to advance
these programs while
             continuing to expand our proprietary discovery activities ...."
March 20, 2018 Press
             Release.
           "For Prothena, it offers the opportunity to work with a premier
scientific partner with
             complementary biological expertise and also provides the resources
and flexibility to
             advance in parallel both the programs within the scope of this
collaboration as well as
             our proprietary discovery activities..." March 20, 2018 Press
Release.
           "...from the Prothena side, this provides an opportunity to
accelerate and expand our
             discovery activities and to do it with expertise and input that we
may not otherwise
             have access to. And so from our perspective, being able to work
with a scientific team
             is actually quite attractive. And obviously from a resource
perspective, we do think
             that this not only advantages the collaboration but advantages our
discovery
             organization beyond the collaboration as well. So we actually see
this as an
 Tran Nguyen
Prothena Corporation plc
April 3, 2019
Page 3
             opportunity to accelerate things that we were looking forward to
working on. And
             actually these are targets that were obviously high on our list
and we were doing an
             extensive amount of work on in any event." - March 20, 2018
Prothena Corporation
             PLC Neuroscience R&D Collaboration Investor Call
3.       With respect to your assertion that the license rights conveyed to
Celgene are not material
         in the context of the contract, please reconcile for us why the
following rights granted to
         Celgene would nonetheless not be viewed as important by Celgene.
           right to access intellectual property for six years through the
requirement to be notified
             on a regular basis of the Company's ongoing research activities,
material
             developments, and new material data or information (see for
example Section 2.2.2.a.
             and b.)
           right to use the Company's licensed intellectual property to conduct
its activities and
             perform its obligations under the Agreement (see for example
Section 7.1.1)
           right to use the Company's licensed intellectual property for six
years for purposes
             other than evaluating whether to exercises its option (see for
example Section 2.9.1)
           right to use for research and development purposes (see page PRTA
1003 of your
             January 22, 2019 response).
4.       Please reconcile for us the statements in your most recent disclosure
in Note 8 to the
         financial statements included in your December 31, 2018 Form 10-K that
the options for
         U.S. Rights and Global Rights represent material rights and the
Company's conclusion
         that it does not have a material distinct performance obligation at
the inception of the
         Collaboration Agreement. In your reply, please provide your analysis
as to whether one or
         more material rights exists, including whether the exercise price of
each option reflects the
         standalone selling price for the goods and services subject to each
option and how you
         made that determination.
5.       Please consider updating your analysis of the performance obligations
in the contract
         based on your responses to the foregoing comments.
6.       Tell us how you considered the guidance in ASC 730-20 in accounting
for this
         arrangement.
        You may contact Tabatha McCullom at 202-551-3658 or Frank Wyman at
202-551-3660,
if you have any questions regarding the comments.



FirstName LastNameTran Nguyen                                  Sincerely,
Comapany NameProthena Corporation plc
                                                               Division of
Corporation Finance
April 3, 2019 Page 3                                           Office of
Healthcare & Insurance
FirstName LastName