Brexit is not solely a UK crisis. Ireland, more than any other member state, stands to be deeply and profoundly impacted by the UK’s departure from the EU. The crisis which Ireland faces is multi-dimensional: economic and political, constitutional and existential in nature. The nature and impact of Brexit changes Ireland’s relationship with the EU, and this is because the UK’s decision to leave the European bloc is also a crisis for the EU. Brexit challenges the balance between intergovernmental and supranational institutions and dynamics in the EU; alters power and policy balances; and adds a degree of contingency to EU membership for all member-states. This article explores the various political, economic and constitutional crises generated for Ireland by the Brexit vote and assesses how Brexit impacts on the Europeanisation and/or de-Europeanisation of the Irish political system and British–Irish relations. This includes an analysis of the extent to which the traditional Europeanised characteristics of the Ireland-EU relationship and the British–Irish relationship will endure or perish within the confines of a post-Brexit EU.