
Btw basically a quick crash course of why Northern Ireland is like that is that the British government hundreds of years ago kicked out a bunch of Catholic Irish in the north and gave the land to Protestant settlers mostly from Scotland. The native Irish got their land rights taken away in the south too but weren’t fully kicked out as much as in the north.
On several occasions the Catholics and Protestants revolted together against the English government (most of the Protestants weren’t Anglican and were harmed by pro-Anglican laws) but once Catholic-centered Irish nationalism started getting underway, the Protestants (most of Scottish or English origins) tended towards loyalty to the British crown.
After the south fully gained its independence, pro-independence militant groups like the IRA remained active in the north and entered into conflicts against loyalist militias and British troops. Lots of bombs, not a good time. So that leaves us to today where the majority-Protestant north remains largely British-identifying and many people are staunchly opposed to Irish unification. Some people can be really bigoted against Catholics and Irish-speakers up there.
Lots of people up there do not consider themselves Irish and feel that their rights wouldn’t be respected in a majority-Catholic state. Meanwhile the Catholics up there have been historically mistreated. And many people in the south want a full unification of Ireland under one flag.