I've actually heard more comments recently that the ROI won't want to take back NI now because it's flourishing in the EU (more so than NI). Okay, Britain built up NI and it has a lot of industry/factories, but a unified Ireland would have to convert to the Euro, design and implement a united kind of emblem for all government agencies etc., plus a lot of other internal changes to how things are run. In other words, it would cost a hell of a lot of money for it to happen.
The UK used to have around 20,000 troops over in NI. I'm not sure of the figure now because they've already withdrawn some troops already. They also run the main airfield/airport, so money for that would have to come from somewhere too.