Whille I have no doubt North Ireland is poorer then England or Ireland, given its political roots as a colony I would be surprised if the colonial overlords doesn't extract tribute in a number of less obvious ways.

If I remember correctly form Late Victorian Holocausts, India was on paper an economic basket case during the colonial era. The real wealth was extracted trough economic regulations that made sure englishmen and English companies made bank, and very little stayed in India.

So short term, North Ireland probably needs those €12 Billion P.a. to clear the budget, but even in the middle term this might shift if less money is then extracted to England (or any of its many banking islands), but stays in the community and can be taxed. And in the longer term, well the republic seems to being much better now then under English rule.