This paper examines Ireland's decision to join the European Union's Permanent Structured Cooperation (PESCO), which forms part of the EU's security and defence policy. PESCO's focus is on defence procurement and defence industry collaboration, two areas where Ireland is not a significant player. Although the decision for Ireland to join provoked some controversy within the Irish Dáil, approval was ultimately supported by the two largest parliamentary parties. This paper first explains the PESCO initiative and then examines its relevance for Ireland. The core question posed is to what extent does membership of PESCO signify a marked departure from a traditionally low salience Irish defence policy and an ambiguous neutrality doctrine? Alternatively, does Irish participation in PESCO reflect continuity, especially given an evolving Irish pragmatic partnership with EU states on defence matters? This paper draws on the literatures concerning small states' defence policies and the domestic limits and opportunity structures for successful multilateral participation by small states. In this regard, we must remember Ireland's systematically low-levels of defence spending and Irish government coalition politics. Both variables suggest domestic constraints for Irish participation in multilateral defence initiatives. Moreover, rather than interpret Irish membership of PESCO as a dramatic departure, this paper argues it reflects continuity in Ireland's somewhat marginal defence policy. This article is based on the author's contribution to a roundtable at the annual conference of the International Affairs Standing Committee of the Royal Irish Academy, titled ‘Multilateralism and Interdependence: Prospects and Challenges’, which took place at the Royal Irish Academy in Dublin on 2 May 2018.