I don't know about Northern Ireland (My understanding is the Irish population always wanted to join the rest of Ireland, and grumblings of succession are nothing new, but the very large British population always held them back
I'm sorry but that is absolutely wrong. Look into Northern Irish loyalism; the divide between Republican Catholics and Northern Protestants goes far deeper than any British meddling. You can bet your house that Northern Ireland will stay in the UK regardless, the possibility of them joining ROI on the Euro ticket is frankly absurd following the decades of strife over there.