I am only going on memories, but I don't think the Glens were ever a ''protestant'' club. East Belfast is/was a Catholic enclave and the fact that Boxing Day Derbies are now Linfield/Glens replacing the old Linfield/Celtic matches suggests that the Glens were the Catholic replacement for Celtic.
Anyway I hope all this has disappeared by now, at least in terms of players. But, as in Glasgow, the ''old'' rivalry may still add to the atmosphere in the stands, even though the players are now more likely to be Muslim than Christian!
Nah East Belfast is a predominately Protestant area. It's West Belfast that has been a Catholic stronghold, and Donegal Celtic who hail from there are a Catholic Club. For as long as I can remember, the vast majority of the Glen's fanbase have been Protestant. In fact a fixture between us and Lisburn Distillery in 2003, was marred by sectarian induced crowd trouble between their Catholic supporters and our Protestant ones.
Glentoran are a Protestant club, but the difference between Linfield and us, is that Glentoran have had a more tolerant approach to Catholics in the past, while Linfield have been more fiercely defensive of their Protestant roots, and far less willing to allow Catholics on board.
Religion wasn't factor in Glens-Blues became the Boxing Day derby fixture. We weren't a catholic replacement for Celtic, we were a footballing one. Without Celtic out of the pixture, it was clear that the Blues and the Glens were now easily the two biggest clubs in the land with the most supporters, so it was a natural choice. Donegal Celtic hadn't been founded then, and the Catholic club Cliftonville from North Belfast weren't a major force in N.Irish football at the time.