Irish Catholics became a solid voting bloc in the 1820s as part of the campaign for the right to sit in parliament (IE 'emancipation'). Daniel O'Connell was primarily responsible for creating the mass campaign. The voting bloc fragmented somewhat between the 1840s and 1870s but then coalesced around the Home Rule party. Irish Catholics remained solidly nationalist but did often follow different parties especially between 1891 and 1901 and in 1918. After independence, the Catholic vote dispersed between Fianna Fáil, Fine Gael and others as all parties essentially supported the Church's social and educational agenda.

In Northern Ireland (post 1921) Catholics voted solidly for nationalist parties. Traditional nationalist politics was quite conservative and clerical but shifted to the left in the 1970s when the SDLP became dominant.

Can't speak for the rest but Northern Ireland has always had deeply set political divides based heavily around religion, with the protestant community almost solely voting for the unionist (support bring in the UK) parties and Catholics voting for nationalist (support leaving the UK and integration into the republic of Ireland) parties, for myriad reasons including English oppression (perceived and actual), religious suppression and (in the latter half of the 20th century) completely uneven civil and voting rights.

Currently writing a lab report up (yay physics degree!) so if you want sources/further elaboration gimme a shout and I'll get them up