The problem with that is that the government of Ireland cannot admit to its population that it cannot afford to take on the burden of Northern Ireland from the UK (although its currently in the best position it has even been to do so).   Northern Ireland costs the rest of the UK about £7bn a year. Even with the drop on value of GBP that is about EUR 7.7bn.  Irish GDP is EUR 340bn, and at the moment its basically spending neutral so does not have a budget deficit.  Uniting Ireland at the same level of support for the north as the British government supplies Northern Ireland, immediately gives Ireland a budget deficit of circa 2% (which is below the EU limit so that is ok)  and gives Dublin the security and sectarian headache that is Ulster.