There's a distinction between maintaining the Union, and a United Ireland. Once there ceases to be a majority in Northern Ireland in favour of staying in the Union, the rest of the UK is free to exclude Northern Ireland. Nothing can stop that - it's not possible to bomb yourself into union.

On the other hand a sizeable minority opposed to going into a United Ireland would be able to make life very difficult.

I envisage that some kind of federal structure is most likely, as the only hope for squaring the circle.

Ian Paisley would go down great in the Dail. If he could bring himself to enter it. He'd be getting on a bit by 2016 though.