...At the moment something significant would need to happen for that to be the case: NI would need to cede from the UK and carry a new referendum or the EU would need to legislate to identify NI (and by dint of that Scotland, England and Wales) as a country with its own rights of representation at the EU. Presumably for the latter any country ceding from the EU in such a way would seek to inherit the UK's membership deal in some form - that isn't a given.
The EU have already said they'd accept NI as a part of The Republic if it was to be reunited. As for the rest, I'm not close enough to the events anymore to be fully aware, just relying on a few folks' occasional tweets from on the ground about current developments.
Frankly, I hope that they can discuss this and come to an agreement because no one wants a hard border there again. There has been a lot of bloody-mindedness in the past, something also prevalent in Scotland which also has a similar divide behind it all. Scotland differs though in that it has valuable resources at its disposal, NI doesn't have so much anymore.
I think up until last week, the issue of Brexit was more firm in this respect. Now though, that might not be so certain. I know JC has proven dogged about continuing with negotiations but the Labour Party have voted to backtrack if the deal isn't favourable. He doesn't want to lose Scotland, once again due to the revenue that he'll then have at his disposal. I'm not so sure about NI.
Of course, a vote or possibly a referendum there would need to take place, as you said. A hard Brexit is possibly the easiest scenario toward unification, which the ECJ will not allow May or her successor to deny them. The Unionists of course will need to be persuaded and they'll probably feel secure in their members concerning an outcome to remain as a part of the UK. In a referendum though people from the north who remember how little London used to give them by comparison to the EU projects and development programmes and how life is now, they'll help swing it. So, call me a romantic but I have a good feeling for Ireland, at last.