The Northern Irish who identify as British would not be very happy there. Pretty sure they would not accept this.
Maybe one solution would be to have a county-by-county referendum, where each county in NI gets to decide whether it wants to stay in NI or instead join Ireland. This would make NI smaller, but would concentrate the UK loyalists in it, and the number of Republicans (the ones who want to rejoin Ireland) would decrease a lot (many would be in the counties that would leave, which are probably mostly right on the border, and some might even move), so maybe re-establishing a hard border would be more palatable then.