The actions of Irish nationalists in Britain are often characterised as a 'sideshow' to the revolutionary events in Ireland between 1912 and 1922. This original study argues, conversely, that Irish nationalism in Britain was integral to contemporary Irish and British assessments of the Irish Revolution between the Third Home Rule Bill and the Anglo-Irish Treaty. Darragh Gannon charts the development of Irish nationalism across the Irish Sea over the course of a historic decade in United Kingdom history – from constitutional crisis, to war, and revolution. The book documents successive Home Rule and IRA campaigns in Britain coordinated by John Redmond and Michael Collins respectively and examines the mobilisation of Irish migrant communities in British cities in response to major political crises, from the Ulster crisis to the First World War. Finally, Conflict, Diaspora, and Empire assesses the impacts of Irish nationalism in metropolitan Britain, from Whitehall to Westminster. The Irish Revolution, this study concludes, was defined by political conflicts, and cultures, across the Irish Sea.