when we refer to anything irish it's celtic pronounced with a "k", but the Boston Celtics are with an s sound

Answer 1

The "c" at the start of "celtic" can be pronounced soft, like an "s", or hard, like a "k". The most common convention is to always pronounce it with a hard "c" ("keltic") except when using it as a proper noun (e.g. Celtic Football Club, Boston Celtics, The Anglo-Celt newspaper).

In Irish, "c" is always pronounced hard, like the letter "k" which is never used in Irish words. The Greeks were the first to write about the Celts, using the word "Keltoi", which suggests that the hard sound is also historically accurate.

So.....Proper nouns (s) adjectives (k) or for those who just want to be historical and ignore convention... (K)

Answer 2

That's because when you pronounce it with a soft "c" as in selt, it means "stone". So the team is named the Boston Stones, presumably because they're so hard to beat.

A Celt with the "k" sound is anyone of Gaelic origins and includes the Irish, Scots, Welsh and some natives of Brittany.