Common membership of the EU has been a background assumption of the peace process in Northern Ireland. While it is correct to note that special arrangements such as the Common Travel Area (CTA) predate EU membership, Brexit creates new and distinctive challenges for Northern Ireland/Ireland. For the first time, the relationship between the UK and Ireland will diverge on EU membership. Northern Ireland is the only part of the UK (bar Gibraltar) with a land border with an EU member state, giving rise to a widely recognised set of unique circumstances. The intra-Ireland border will become an external border of the EU.