It was never about religion, from the Republican/Nationalist side.
Among their greatest heroes were Protestants and Nonconformists...
Henry Grattan
Napper Tandy
Thomas Russell
Lord Edward Fitzgerald
Wolfe Tone
Robert Emmett
Isaac Butt
Charles Stewart Parnell
Sir Roger Casement
Erskine Childers (IRA)
Constance Markievicz (nee Gore-Booth, the first elected woman MP, Sinn Fein, 1918)
Robert Barton (IRA, first Irish government, 1922)
Ernest Blythe (IRA, first Irish government, 1922)
Douglas Hyde (the first President of Ireland)
Seán Mac Stíofáin, born John Edward Drayton Stephenson in England to an English Protestant father and a mother of Ulster Protestant and Unionist. [Leader of the IRA in the 1970s]
and countless more.
From memory, a slight majority of the Irish delegation (IRA) who came to Downing Street in 1921 to negotiate with Lloyd-George and Churchill for independence were non-Catholics...
A pretty good 1991 drama from RTE/Thames
"The Treaty", the strongest holdouts were Barton and Childers...
https://www.youtube.com/watch?v=RUfr0FgZz_8