Actually, I was typing my post while Adam was sending his, so I didn't see it until afterwards. It seems only to have been since the "Celtic Revival" began to get off the ground during the 19th century that people started to refer to themselves as "Celts" (often, at that time, spelled "Kelts"); the word as used nowadays in Gaelic to mean "Celt" rarely appears in older Gaelic dictionaries with that meaning, and explanations to the effect that it "means" something in Gaelic seem to be based on coincidence of orthography rather than any actual etymological relationship; it does appear to be a recent loan-word in the context of its modern usage. If that is indeed the case, conventions of Gaelic pronounciation would be irrelevant to the discussion.