First thing they would need to discuss is how they will fill the £10 billion fiscal gap from the UK treasury if Northern Ireland united with the Republic. For context, that is more than ten times their defence budget and about 2/3 of their health budget.
Which means with unification the Republic would either have to make savage cuts to Northern Ireland’s public services or raise the crap out of the rest of the country to fill the shortfall.
Alternatively, maybe they can go cap in hand to Biden and ask him if the Irish Community will have a whip round to fill the void left behind by the British taxpayer...
So long as there’s a European Union, there’s no such thing as “unification”. Aside from the Unionists not having it, Ireland loses her identity forever no matter what and becomes a province once again. (This is what Nicola Sturgeon wants to do to Scotland too, i.e. destroy its identity.)