The Presbyterian records do exist (well mainly exist!) but many of the churches have closed.
My g-grandfather was the minister at the Abbey St Presbyterian Church, which is now closed, but I did manage to fine that the records had been transferred to another Presbyterian church which is still operational, and they were kind enough to extract the information that I needed.
You would need to find what was the original Presbyterian church(es) in St Thomas and St George. If there is a Dublin resident reading this they can give you that information much quicker than I can research it! Then if it is closed email or phone a neighbouring Presbyterian Church to see who held the records.
I should add that 100 years ago in Dublin (and in Scotland) there were various "flavours" of Presbyterianism, as every so often there was a schism on some deep point of principle, and half the congregation would move down the road and set up a new church.
