When I was in college and took some Irish Gaelic, our professor instructed us that the Irish Gaelic does not have a soft 'c'. As such, I was taught to pronounce the word keltic. That said, however, I'm also not certain if I ever heard him use the word, and if its origins are in fact Greek, who knows how to pronounce it in Irish? I've always been inclined to pronounce words we get from other languages the way they are pronounced in those languages, so if it was pronounced with a hard 'c' in Greek, I'd be inclined to pronounce it with a hard 'c' in English. English borrows so many words from other languages, though...

I do also remember that our professor repeatedly reminded us that pronunciation in different parts of Ireland could be very different. He reminded us of this particularly when referring to the spelling practice of lenition, which does things to how one pronounces lots of consonants. I don't remember right now if a 'c' ever got lenited (I don't think so), but if it didn't, then its pronunciation would always be hard. At least in the dialect of Irish he was teaching us, which was what he had learned--but I can't remember where he said he learned it (his parents sent him from Co. Waterford to somewhere else to learn Irish, but I don't remember where).

So I'm not much help either, except to say I've always used a hard 'c'.
:)
Monica