I'm very greatly to the government for giving me (2nd generation immigrant) a passport which is highly likely to retain its EU privileges
Of course, the concept of a United Ireland is very appealing for sentimental reasons.
The insurmountable barrier I see, is that Northern Ireland is not economically self supporting. For example, great jobs are quite thin on the ground. The UK civil service is one of very few big employers. What could possibly replace those jobs?
What would the Republic have to gain from taking in people who still defend their "right" to march along routes, singing sectarian songs, through areas where the people living there now are, to put it mildly, not absolutely in sympathy with their cause.
It's not just so easy.
Perhaps the EU would stump up a support package to help the economy.