The Democratic Unionist Party (DUP) gained an unexpected foothold at the heart of the British political system following the 2017 UK general election. Political arithmetic compelled the then Prime Minister Theresa May to enter a Confidence and Supply Agreement with Northern Ireland’s ten DUP MPs in order to shore up her minority government. The timing of the DUP’s positioning at the UK’s constitutional centre coincided with the early phase of the Brexit process and afforded the small Northern Ireland political party a degree of influence as the UK struggled to agree the terms of its departure from the EU. This article provides some analytical clarity as to how and why the DUP unexpectedly came to play a leading role in Brexit’s complex and dramatic political theatre. Drawing on interviews with senior DUP figures, opposing political parties, civil servants and political commentators, this article demonstrates the hollowness of the DUP’s Brexit position, and points to ways in which the party’s influence over the UK’s approach to the Brexit negotiations undermined relationships in Northern Ireland between unionists and nationalists, between North and South (on the island of Ireland), and between Ireland and the UK. The research reveals that Brexit has precipitated (a return to) a disruptive Unionist politics which is defined by a profound and destabilising ontological insecurity and a fear of being ‘sold out’.