Ireland’s rapid economic growth from the mid-1990’s combined with the opening up of the Eastern European labour market, led to Ireland moving from being a country of net outward migration to becoming one of net inward migration at a speed that was unprecedented. This created a major challenge to the Irish trade union movement which was already operating in a context of the erosion of traditional forms of employment and employment relations and a declining membership. This thesis will explain the impact of migration to Ireland on trade union policies, rhetoric, attitudes and organisational approaches. It foregrounds the influence of the trade union movement on the migration policy environment and investigates the trade union response to labour migration in terms of changing forms of employment relations as a result of outsourcing, the increasing presence of labour market intermediaries and the growth of the informal sector and irregular forms of employment. The thesis is situated in the context of the international debates around the economic, political and social modalities of trade union action. The Irish case will, in its turn, illuminate those debates and posit a new and emerging model of unionism based on a particular combination of modalities. The primary contribution of this thesis is to the trade union revitalisation debate. A comparative diachronic methodology illuminates the role that the key tipping points of the two major industrial disputes at Gama and Irish Ferries played in that debate and in the subsequent reconfiguring of capital, labour and state relations. I posit, in conclusion, that the emerging Irish model of union organising is a distinct variant on the international organising model, based primarily on the organisation of migrant labour and internal union dynamics.