From Rag's piece,
"The prospect of a no-deal exit and a hard border would play to Sinn Féin’s political aims of a united Ireland, with 53% of Catholics in favour if this was the outcome of the Brexit negotiations."

SPB,
Also there is the other issue that to allow one part of the UK to become part of ROI, without extending the same right to the rest of the UK would be discriminatory.

No. It was agreed years ago that if the North voted by at least one percent to unite, it would happen. Other parts of the UK can vote for independence, but there is no other part showing any interest in joining Ireland.