Having lived in Belfast for three years in the early 90s, when the troubles were pretty full-on, I can say that you need to experience it to fully understand the depths of distrust and hatred between certain factions over there. Whilst the Good Friday agreement bought and end to the senseless killings of civilians and security forces to an end, these people still have access to all the weapons they need, and still possess the knowledge required to start up a full scale campaign again. There are still large divides, and whilst these groups are "disarmed" they still carry out attacks, normally punishment beatings. The groups run a lot of illegal activities, during the troubles this was to raise funds for weapons but now it lines their pockets - I still have friends over there, and normally go over once or twice a year for work.
Whilst a united Ireland may suit the Republican cause, be it the IRA or any of the smaller terrorist groups, I would imagine you would see renewed attacks by the Loyalist terror groups UVF, UDA, UFF or even the Red Hand Commando, who would be wholey opposed to allowing NI to break away from the UK.
The whole thing is a clusterfuck, and everything should be done to prevent a hard border being re-instated.