"Why should all Ireland not be part of England?"   That's a bit like saying "Why should my leg not be part of my arm?" If my arm was part of my leg, it wouldn't be a leg.

If most people in Wales or Cornwall wanted to secede from England I doubt very much if anything would be done to stop them. The Westrminster Goivernment has learnt better than to try to resist that kind of thing by force. When a majority in Northern Ireland as a whole decide they've had enough of the English connection, the same will apply.