It’s a shared province don’t forget.I know. In particular, laws passed in the eighteenth century were like a kind of apartheid against Catholics. They've all been rescinded now, though, and most Ulster Protestants want to be part of the United Kingdom still.
Even the republican first minister designate.It goes back a lot further than the 1960s! Which is why they sought independence from the British in 1921. The problems between the monarchy and Irish go back a few hundred years.
That said I am in NI, and haven’t met anyone yet who is anything but respectful about the Queen dying, even if they aren’t a royalist.