This volume explains the genesis and development of the nexus between radical Basque nationalists and Irish republicans, how they have learnt from each other historically, and how they have utilised this relationship, at times, to their benefit. From medieval tales of shared origins to the violent conflicts largely wrought by ETA and the IRA, the Basque Country and Ireland have long been associated in popular imagination. Despite this, little is known of historical Basque-Irish relations and, in particular, the web of party-political, military and social movement connections between radical Basque nationalists and Irish republicans since the Irish Revolutionary Period (1916–23). Drawing on extensive archival research undertaken in Spain, Ireland and the UK, and more than 70 interviews conducted with politicians, former paramilitaries and grassroots activists, this is the first study to comprehensively document and analyse the emergence, evolution and implications of this mythified transnational relationship. Radical Basque Nationalist-Irish Republican Relations: A History will appeal to students and scholars of Irish republicanism, Basque nationalism, terrorism studies and social movements studies, as well as those interested in the contemporary history of Western Europe’s two most volatile regions.