Federalism has a rich, if chequered, history within the political discourse of the British and Irish Isles. This article examines the political thought of one much misunderstood advocate of federalism, Isaac Butt, and the debate that his proposal to transform the British constitution prompted. It seeks to recast Butt, the founding father of Irish Home Rule, as a champion of the Union. Historians of Ireland, Britain and federalism, like many of Butt’s contemporaries, struggle to position him within the spectrum of nineteenth-century political thought. The most important work on Isaac Butt and the early Home Rule movement in Ireland remains David Thornley’s study from 1964, which skews its subject matter by framing Butt as representative of ‘a curious imperial nationalism’. This article argues that there was nothing ‘curious’ about Butt’s political thought, and that his conception of a distinct Irish identity within the broader rubric of the Union has been under-appreciated. For Butt, Home Rule was the mechanism to bind Ireland to Britain and its Empire, thus resolving the ambiguities of the Act of Union of 1800. Butt’s formulation of Home Rule in 1870 envisaged a federalised United Kingdom, which would have weakened Irish exceptionalism within a broader British context. The article positions Butt as representative of a constructive national unionism, and explores the implications of this for Ireland’s relationship with British identity and the Union during the nineteenth century.