Ireland’s relationship with the European Union has been determined by the behaviour, actions and discourse of political parties. This book examines this impact through an in-depth analysis of the Europeanization of party politics in the Republic of Ireland and Northern Ireland. First, it presents original research on cross-cutting issues that have featured in political debates about European integration, including referendum campaigns on EU treaties, Irish neutrality and party policy positions on the EU. Secondly, it is the first book of its kind to examine in detail how each of the main parties on the island of Ireland has adapted to EU membership. In doing so it both tests the thesis of ‘Europeanization’ and deepens understanding of the impact that EU membership can have on national and sub-national party politics. What this study reveals is that, while Europeanization is clearly evident in all parties in Ireland, including those most critical of European integration, its influence has been strictly curtailed. We argue that the effects of Europeanization in Irish party politics have been limited by enduring resistance to – and conditions placed upon – EU influence in particular policy areas, the importance of pragmatism and (sub-)national priorities in shaping parties’ approaches to European integration and the fact that engagement with the EU continues to be a predominantly elite-led process. This book was published as a special issue of Irish Political Studies.