I'm really worried about northern Ireland, apart from border controls, both the unionists and the republicans are going to dig in and go back to the bad old nationalistic days, bombings from the IRA, child murder by the British Soldiers.
The best they could hope for at the moment is declaring themselves an independent nation state, which would both be in the commonwealth and in Europe, which would leave exactly no one happy: The people who want a 32 county Republic would be pissed off, the ones who consider themselves British would be pissed off, and everyone in the UK would be pissed off because it would be a back door for migration into the UK.