Actually they’re Catholics. Sinn Fein in Gaelic means “We Alone’’.
Sinn Fein was never "Catholic," but it was the party of choice for Catholics for many years. Since they have recently embraced abortion and same-sex marriage, they are now being disavowed by orthodox practicing Catholics-- which are becoming rarer in Ireland, apparently.
Priest in N Ireland cancels Sinn Féin meeting at parish hall over abortion
Only on paper. No "Catholic" political party would support abortion and gay marriage.