The vote by the electorate of the United Kingdom to leave the European Union in 2016 was one in which the impact of Brexit on the island of Ireland and on UKIrish relations hardly figured. Within months, however, the ‘Irish border problem’ was centre stage. The deterioration in UK-Irish relations in the 2 years following the referendum was profound and marked the first stage in the potential unravelling of the deep interdependence which had come to characterise relations between Dublin and London by virtue of their shared membership of the European Union since 1973. A significant ‘reverse asymmetry of power’ emerged from the United Kingdom’s relative isolation in the Brexit negotiations and Ireland’s privileged position as an European Union insider. In an increasingly turbulent international arena, the retreat from integration set in train by Brexit also threatened the Good Friday Agreement and the institutions and processes put in place to manage North–South and East–West relations after 1998.