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.

	{ Thanks for clarification to Michael Ruddy. }