I think there has to be a number of factors to cater for before a united Ireland becomes a reality.
1. There must be a successful 'pro' referendum in the North.
2. There must be political agreement in the South.
3. The British government must agree.
4. There will need to be an EU finger in the pie.
Because of the young age of Northern population, a majority of Catholics, a desire to get over the protocol, a wish to join the EU will all contribute ultimately to Unification.