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CORRESP

 FOIA CONFIDENTIAL TREATMENT REQUESTED PURSUANT TO RULE 83

Prothena Corporation plc

 77 Sir
John Rogerson’s Quay, Block C

 Grand Canal Docklands

Dublin 2, D02 T804, Ireland

 January 22,
2019

 Via EDGAR Transmission and Overnight Delivery

U.S. Securities and Exchange Commission

 Division of Corporation
Finance

 Office of Healthcare & Insurance

 100 F
Street, N.E.

 Washington, DC 20549

 Attention:

 Frank Wyman

 Tabatha McCullom

 Re:

 Prothena Corporation 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

 Ladies and Gentlemen:

 Prothena
Corporation plc (the “Company”) submits this letter in response to comments received by letter dated December 19, 2018 from the Staff of the Securities and Exchange Commission. A courtesy copy of this letter is being submitted to the
Staff by overnight delivery.

 Each of the Staff’s comments is reproduced below in bold. The Company’s response follows each such Staff comment.
Capitalized terms used but not defined herein have the meanings ascribed to them in the Master Collaboration Agreement by and between Prothena Biosciences Limited (“Prothena”) and Celgene Switzerland LLC (“Celgene”) dated
March 20, 2018 (the “Agreement”) to which the Staff’s comments relate.

 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

 [***] Certain information in this
document has been omitted and filed separately with the Securities and Exchange Commission. Confidential treatment has been requested by Prothena Corporation plc with respect to portions of this letter.

Confidential Treatment Requested by Prothena Corporation plc

 PRTA-1001

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 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.

 Company Response:

The overarching intent of the Agreement was to convey options to Celgene to obtain commercial rights to antibodies that may be developed under
three research programs, and the consideration associated with that arrangement has been allocated to those option exercise rights. In order to convey those options, certain arrangements related to intellectual property were agreed upon to protect
both parties and to enable progress of those research programs to a point where Celgene may elect to exercise its option rights. However, neither of the limited licenses granted under Article 7 of the Agreement convey distinct value apart from the
rights of Celgene to opt into future development opportunities by exercising an option. Specifically, those two licenses identified by the Staff permit Celgene certain access rights that may contribute to an informed decision-making process with
respect to whether to exercise their option rights while ensuring that neither party’s intellectual property is compromised. Following are additional details with respect to each of the two licenses.

Limited License to Conduct Activities Under the Agreement: The Company concluded that this limited license, under Section 7.1.1 of
the Agreement, did not constitute a distinct or material performance obligation under the Agreement.

 Prior to Celgene’s exercise of
an IND Option for a Program under the Agreement, Celgene’s license rights to the Prothena IP under Section 7.1.1 of the Agreement are essentially limited to [***] as set forth in Section 2.4 and to using certain materials which may be
transferred to Celgene by Prothena for certain specified purposes as set forth in Section 2.9. The [***] under Section 2.4 of the Agreement [***]. The materials that may be transferred by Prothena to Celgene in accordance with
Section 2.9 of the Agreement may only be used by Celgene to determine whether a given Collaboration Candidate may be a Lead Candidate, to evaluate whether to exercise an IND Option or a Phase 1 Option for a Program, or to [***]. These purposes
are very limited. In substance, this simply provides a means for Celgene to receive information that may

 [***] Certain information in this
document has been omitted and filed separately with the Securities and Exchange Commission. Confidential treatment has been requested by Prothena Corporation plc with respect to portions of this letter.

Confidential Treatment Requested by Prothena Corporation plc

 PRTA-1002

support a future purchasing decision – that is, it provides information that may be relied upon by Celgene for purposes of deciding whether it should buy distinct goods and services in the
future. Accordingly, this license does not constitute a transfer of a good or service from which Celgene could generate economic benefit and therefore is not distinct.

We also note that under Section 7.1.1 of the Agreement, Prothena provides Celgene with only a limited license to Prothena IP for a
limited purpose for a fixed term. The license is non-exclusive and only for Celgene to conduct its activities and perform its obligations under the Agreement. We view this limited license as immaterial in the
context of the Agreement given the narrow scope, limited use and limited term of the license.

 Limited License for Research and
Development Purposes: Similarly, the Company concluded that this limited license, under Section 7.1.2 of the Agreement, did not constitute a distinct or material performance obligation under the Agreement.

Importantly, [***], which means that [***]. Moreover, at inception of the Agreement [***] as by the very definition of such terms under the
Agreement, [***]. Additionally, [***]. Accordingly, this license does not constitute a transfer of a good or service from which Celgene could generate economic benefit and therefore is not distinct.

We also note that under Section 7.1.2 of the Agreement, Prothena provides Celgene with a limited license for a limited purpose. The
license is non-exclusive, is limited to [***], and is only for research and development purposes. We view this limited license as immaterial in the context of the Agreement given the narrow scope, limited use
and limited term of the license.

•

 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.

Company Response:

Two Licenses: For the reasons set forth in our response to the foregoing comment, the two licenses did not constitute distinct or
material performance obligations. Rather, they help position Celgene to make future purchase, i.e., option exercise, decisions.

Participation in Joint Steering Committee: Prothena’s responsibilities to participate in the joint steering committee (the
“JSC”) constitute a performance obligation that is immaterial.

 [***] Certain information in this
document has been omitted and filed separately with the Securities and Exchange Commission. Confidential treatment has been requested by Prothena Corporation plc with respect to portions of this letter.

Confidential Treatment Requested by Prothena Corporation plc

 PRTA-1003

 The JSC has limited governance responsibilities and essentially serves as a mechanism for
Prothena to communicate and discuss with Celgene any research activities that Prothena decides, at its discretion, to undertake. Additionally, Prothena’s responsibilities
vis-à-vis the JSC operate only to ensure that Celgene remains informed, and are substantively limited to making decisions regarding the research activities which
Prothena chooses to undertake at its discretion.

 To further assess the materiality of our obligation to participate in the JSC, we
estimated the standalone selling price of that JSC participation and determined that no observable or contractual price existed. The Company therefore utilized the expected costs plus margin approach to determine the estimated standalone selling
price of the JSC deliverable. In determining the estimated standalone selling price of the JSC deliverable, the Company considered factors such as the number of Company representatives participating on the JSC, the annual number of required JSC
meetings, the assumed meeting length in hours, and the expected term of the JSC. The Company assumed the JSC would continue to function through the term of the Agreement (approximately six years). Based on this analysis, the Company determined the
estimated standalone selling price of the JSC deliverable is approximately $0.1 million to $0.4 million.

 The Company therefore
concluded that the impact on the allocation of arrangement consideration of including the estimated standalone selling price of the JSC deliverable, calculated assuming the high end of the range of estimate selling price noted above, is immaterial
from both qualitative and quantitative perspectives.

 Conduct of Research Activities: Prothena may, in its discretion, conduct
research activities under the Agreement. To the extent it undertakes those activities, Prothena’s essential responsibilities vis-à-vis the JSC are to provide
it with regular updates for the purpose of informing and enabling discussions with Celgene and make certain decisions regarding such activities.

The JSC may perform certain specified functions, subject to, generally speaking, [***]. Importantly, the JSC [***]. Therefore, Prothena
has no material performance obligation in its conduct of research activities. Rather, the intent of the Agreement is to convey (for consideration) a right for Celgene to purchase future licenses and services from the Company. The licenses and rights
to access the status of work conveyed at inception of the arrangement support Celgene to make informed future purchasing decisions based on the status of each program but do not compel the Company to perform any additional services. Because these
access rights simply allow for sharing of information that would be considered in the context of future purchasing decisions, we do not believe they themselves represent distinct performance obligations but rather are part of the ongoing selling
effort. That is, Celgene needs ongoing access to information via the JSC in order to decide whether or not to exercise an option.

 [***] Certain information in this
document has been omitted and filed separately with the Securities and Exchange Commission. Confidential treatment has been requested by Prothena Corporation plc with respect to portions of this letter.

Confidential Treatment Requested by Prothena Corporation plc

 PRTA-1004

 Prosecution, Maintenance and Enforcement of Certain Patents: Consistent with ASC 606-10-55-64A, Prothena concluded that its responsibilities with respect to prosecution, maintenance and enforcement (which includes
defense) of Prothena Collaboration Patents and Program Patents are not a promised good or service.

 Prior to Celgene’s exercise of an
IND Option, Prothena has the sole right, but does not have any obligation, to enforce the Prothena Collaboration Patents or Program Patents (if any). Additionally, Prothena’s limited obligations to prosecute and maintain the Prothena
Collaboration Patents and the Program Patents (if any) in the Primary Patent Countries do not mean that Celgene has a license to those Patents. As noted above, unless and until Celgene exercises an IND Option, its license rights are very limited,
and these prosecution and maintenance obligations therefore provide no good or service – nor would they generate any economic benefit to Celgene – and are merely administrative activities that Prothena would have performed irrespective of
this Agreement.

 For these reasons, we also view Prothena’s obligations in this regard as immaterial to the Agreement.

•

 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.

 Company Response:

As noted above, Prothena may conduct discovery, pre-clinically develop, conduct further
development activities, provide a Research Plan, etc. under the Agreement with respect to a Program. These and other research and development activities are not an obligation of Prothena; they may be undertaken by Prothena in its sole discretion.

 The referenced statement by Prothena – that it “is responsible for all research and development activity through completion of
Phase 1 clinical studies of products in each program, unless Celgene elects otherwise at its cost” – only applies following the exercise by Celgene of its IND Option for a particular Program.

In order to correct and make this more clear in our disclosures, we will revise as follows the relevant statement in Note 8 of the Notes to
Condensed Consolidated Financial Statements included in our Annual Report on Form 10-K for the period ended December 31, 2018 (and in future filings):

For each such program, Celgene has an exclusive right to license clinical candidates in the U.S. at the IND filing and if exercised, would
also have a right to expand the license to global rights at the completion of Phase 1. Following the exercise for U.S.global rights, Celgene would have

 [***] Certain information in this
document has been omitted and filed separately with the Securities and Exchange Commission. Confidential treatment has been requested by Prothena Corporation plc with respect to portions of this letter.

Confidential Treatment Requested by Prothena Corporation plc

 PRTA-1005

decision making authority over all further global clinical development activities and all regulatory, manufacturing and commercialization activities in the U.S. The
Company would beis responsible for certain of those activitiesall research and development through completion of Phase 1 clinical studies of products in each such program for which
Celgene exercises its US Rights, unless Celgene elects otherwise to do so at its cost.

•

 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.

Company Response:

The JSC [***]. In accordance with the Agreement, Prothena will inform Celgene via the JSC of any discovery, preclinical and clinical
development activities that Prothena undertakes at its discretion. This information will facilitate any activities that the JSC may undertake and decisions, if any, that it determines to make. Importantly, the JSC has no authority beyond the
enumerated funct