For those who spoke on behalf of Leave voters, the result on 23 June 2016 meant the people of the United Kingdom were taking back ‘control’ or getting their ‘own country back’. However, two parts of the United Kingdom did not vote to leave: Scotland and Northern Ireland. Here, the significant counterpoint to ‘taking back control is “waking up in a different country”‘, and this sentiment has unique political gravity. Its unique gravity involves two distinct but intimately related matters. The first concerns politics of identity. The vote was mainly, if not entirely, along nationalist/unionist lines, confirming an old division: unionists were staking a ‘British’ identity by voting Leave, and nationalists an Irish one by voting Remain. The second concerns borders. The Good Friday/Belfast Agreement of 1998 meant taking the border out of Irish politics. Brexit means running the border between the European Union and the United Kingdom across the island as a sovereign ‘frontier’. Although this second matter is discussed mainly in terms of the implications for free movement of people and goods, we argue that it is freighted with meanings of identity. Brexit involves a ‘border in the mind’, those shifts in self-understanding, individually and collectively, attendant upon the referendum. This article examines this ‘border in the mind’ according to its effects on identity politics and the constitution, and their implications for political stability in Northern Ireland.