The Republican vs. Unionist divide is always portrayed as Catholic vs. Protestant. But "protestantism" is a pretty big tent. Did any protestant groups side with the IRA in an "enemy of my enemy" sense? How about for purely nationalist reasons?
There were ties between Irish revolutionaries and various communists, so I presume at least a few dialectic materialist atheists were floating about, but Im most curious if some Free Baptists or whomever decided to be so free that the ended up on the other side altogether.
There were Jews in the IRA. See this site, for example. I have a family connection with this, but it is anecdotal, so I won't elaborate except to say that Jews were very much part of the Easter Rising in 1916 and the struggle in 1920s.
I'm not sure if you mean the IRA of the 1919-21 revolution or the 1969-94 Troubles. In either case there were protestant individuals involved but no churches supported them (not even the Catholic church). From a protestant point of view, there was little reason to leave the UK which is a pretty religiously free society for a largely Catholic united Ireland. Those protestants who were Republican tended to be so for cultural or political rather than religious reasons.