"I think the term 'high' used is the same as that in High Church and was coined by the protestant Englishified antiquarians in Dublin"

I don't think so. My understanding is that the term was first used for free standing crosses as opposed to the earlier crosses carved on stone slabs. (Malcolm Seaborne -Celtic Crosses of Britain and Ireland)

Another theory is that high crosses marked the highway (highway derives from "hay way" - the road by which hay was transported by wagons) This seems unlikely, but terms come into use and are soon mis-used.