True but did the Protestants and unionists of Southern Ireland agree to the treaty? Where have all of the Protestants of Southern Ireland gone? Many Protestants in the early Free State were targetted for "revenge" attacks in response to so called Catholic persecution in the north. They hardly exist in the ROI today.
They weren't even consulted as far as I know.
Protestants in the South were only a tiny minority then. The South was depressed and hardly industrialised as opposed to the industrialised, protestant North. There was little reason for protestants to live in the South.
The whole issue is not religious believe it or not. It's simply that protestants are pro-foreign rule, non-Irish and middle class and catholics are republican nationalist, working class. Protestants had disproportionate economic clout, too.
Hard to believe now but in Australia, up until the 60s Catholics voted Labour and protestants voted conservative. It was very tribal here, but it was socio-economic more than theological.