Since the 1950s, European integration has included ever more countries with ever-softening borders between them. In its apparent reversal of integration and its recreation of borders, Brexit intensifies deep-seated tensions, both institutional and territorial, within and between the constitutional orders of the United Kingdom and Ireland. In this book, leading scholars from the UK and Ireland assess the pressures exerted by Brexit, from legal, historical, and political perspectives. This book explores the territorial pressures within the UK constitution, connecting them to the status of Northern Ireland before exploring how analogous territorial pressures might be addressed in a united Ireland. The book also critically analyses the Brexit process within the UK, drawing on Irish comparative examples, to assess unresolved tensions between popular mandate, legislative democracy, and executive responsibility. Through practical application, this book explores how constitutions function under the most intense political pressures.