Historically it's always boiled down to religious afiliation and alegiance to the crown, nowadays neither is that important in peoples everyday lives, without the whole protestant/catholic thing as well as the royal family (the two go hand in hand) I don't see why there couldn't be a union between England, Wales, Scotland and the whole of Ireland.
Ach sure hi while we are at it why can't the US be joined with britain like the good old days. National self determination ever occur to you?