Even beyond the economics, people in the Republic would have to ask themselves if they really want the version of a United Ireland that would be on offer. It's not like a United Ireland would just be the Republic with NI added in, it would likely have to involve great concessions on the Republican side to the point where the new state would be partly British.
For example, would people from the Republic be happy with a UI where the tricolour is gone, maybe a Union Jack is incorporated into the new flag as with Australia and New Zealand, a new constitution is in place, maybe part of the commonwealth, maybe a push for a monarch as head of state as with Canada, Australia and New Zealand, maybe unionists guaranteed seats in whatever new version of the Dail exists, maybe a far more fedarlised structure that gives NI more autonomy (at great bureaucratic cost), maybe a new police force, etc.?