It won't happen.
Even discounting the whole clusterfeck that is Catholics vs Protestants and that a unified Ireland could end up setting us back over thirty years in terms of peace treaties and prosperity and calmness between the two nations, the fact is, ROI cannot afford to take in NI.
We're already incredibly reliant on Dublin to fund the rest of the country, which hasn't recovered nearly as well as our prosperous capital. We can't take in a country of 1.8m people who have a far lower GDP than us, whose economy is so reliant on service, and who already require huge EU and UK funding.
Bear in mind that I love the idea of a unified Ireland and I'm sure Sinn Fein and others who love it will blindly campaign for it for the next decade, a bit like Brexit, ignoring the implications and spouting bullshit, but it's just not sustainable for either state. It won't work.
NI is in a far tougher situation than Scotland because they're simply not remotely as wealthy nor do they have a financial hub like Edinburgh, or the infrastructure to go it alone. They need the UK.