"The Prime Minister of Northern Ireland"
The artificial division was already in place and a non-representation puppet state did the thing they were told to do without any input from the affected population. The partition away of part of Ireland, including non-unionist areas, had already happened. The ability of the British government to fabricate history with nonsensical language long predates this ("The British Isles") and continues to the modern day ("confidence and supply").
I mean that one's easy, they wanted to break the Good Friday Agreement and put up a border in Ireland. Didn't work out for them but they managed to break it anyway by going into government with the Tories.