That is simply untrue (and its also downright insulting to the Unionist people). The nationalist Irish members of parliament astutely guided through a home rule bill in 1912 and the British Government accepted it and passed it through parliament. It only required the King's signature to become law. It was the people of Ulster who rose up against the act. See the Ulster covenant.
http://en.wikipedia.org/wiki/Ulster_Covenant
In the second place, why would they want to do what you suggest? Is Northern Ireland so valuable a piece of real estate? Err, no. Its been nothing but trouble. Why would they want to hold onto it? For what purpose? Tyranny for tyrannies sake? Please...
If southern California voted to join Mexico, I don’t think most Americans would view it as a legally binding arrangement and feel compelled to give it up. Would they? If Ireland was part of the United Kingdom and voted to secede, I don't think most Britons would view it as a legally binding arrangement and feel compelled to give it up. Would they?
You see? It's exactly the same argument. You complain that part of Ireland didn't have the right to opt out of independence, but you have no problem that a part of the United Kingdom voted itself out. The British argument is simple and it is clear. Self-determination. If people want out they can go, if they want in they stay. What is your problem with that? Forget how the horrid situation all began. At this instant YOU are the one who is arguing for coercion.
The occupation of Northern Ireland is indefensible.
As it's not an occupation, it doesn't need to be defended.
The people of Ireland (ALL of Ireland) would have supported independence for ALL of Ireland in a landslide. At the time, Northern Ireland was economically worthwhile (the Titanic was built in its shipyards, though Catholics weren’t allowed to work on it - there’s karma for you). It was only when globalization did to their industry what it had done to Britain’s industry that London lost interest in the whole thing.
The “unionist” people are visiting transplants, nothing else; their threats to bomb London if the British withdraw is the reason why it is still occupied.