Bringing the six counties as is into the Republic as it is would just lead to trouble unless handled very well.
A forced united Ireland is not any better than forcing the counties which had majority nationalists back in the 1920s but whose opinion was denied because a 3.5 counties Northern Ireland wouldn't have been viable.
- From 1921 onwards, a sizeable minority were not happy to not being a part of RoI.
- From 20XX onwards, a different sizeable minority were not happy to not being a part of the UK.
Neither is a solutions. Compromise and consensus are what is needed, not a wait until there are 50% + 1 votes for united and alienate 49%.
If an united Ireland comes, I think the Republic should become federal or confederal.
Ireland is a reasonable modern country with a written constitution and STV long used to coalitions so I don't think that would be too much of a problem, but southern politicians would have to seriously talk to unionist and loyalists communities and I think the negotiations would be as hard as negotiating the GFA was.