You mean a Northern Ireland unification referendum I’m guessing?
Firstly, there will be a NI unification referendum at some point in the future…whether it’s directly due to Brexit is doubtful IMO, because it wont happen for many years.
Secondly, if the population of NI vote in a referendum to leave the UK and unify Ireland then that’s their democratic decision. It’s nothing to do with the rest of the UK.
Thirdly, a NI unification referendum would be more divisive than any Brexit referendum. A decision to hold one would not be taken lightly and certainly not at the whim of any English politician.