A heat map of the UK voting for Brexit shows it heavily English and rural in England as well as the rural north east of Northern Ireland [1]
Scotland had only recently declined to become independent but I would bet that too will be revisited.
When Michael Collins signed the treaty, he knew there was no way to keep Ulster in the nation without bloodshed. He got that anyway with the civil war and his death. But in the 1920s all the way through even the 1970s, the notion of protestants becoming part of a Catholic-dominated Republic government was impossible. In a small way I don' t blame them (while still recognizing the blatant bigotry of the Ulster protestantism toward Catholicism).
Michael Collins was largely responsible for the first Irish Constitution for the Irish Free State. [2] It forbade religious discrimination, but the strong hold of the Church on the people would largely circumvent that provision. It got worse when Eamon DeValera came into power. He was responsible for creating a new constitution in 1937 which practically enshrined the Catholic Church as a branch of government and made women second class citizens! In fact, the govt delegate much of education to the Church such that even in 2016, when my wife and I visited Dublin and attended a couple socio-economic-political events, one man at the education event who was Jewish said he had to lie to the local (church run) elementary school that his kids had been baptized in the Church to get them admitted to that school! No protestants were going be put under that regime.
Michael Collins' vision was that if the independent Ireland did the right things for its people, eventually the protestants in the north would find it in their own interests to unite the two parts of Ireland. Little did Collins realize the hold that bigotry has on people.
When my wife and visited Spain in 2002, we took a day drive into Gibralter, a peninsula held by the British for 300 years, having taken it from Spain. Spain has been agitating for its return in the modern era. While there, we saw a poster for a political rally in which the lead speaker was a convicted Ulster Liberation Army terrorist newly released from prison. The ULA was the protestant terrorist group combatting the IRA. This ULA speaker was there to speak to Gibraltans about how to remain British!
But I think attitudes and demographics are changing rapidly. I had an aunt who used to smuggle guns for the IRA in the 1970s. I am 64 years old and expect to see a united Ireland in my lifetime.
[1]
[2]
https://en.wikipedia.org/wiki/Constitution_of_the_Irish_Free_State#:~:text=Shortly%20after%20the%20British%20evacuated,into%20being%20in%20December%201922.
