she may be named after a Celtic goddess...take your guess:
(The place where I found the list of Celtic Goddesses is here, if anyone is interested:
http://www.paralumun.com/celticgoddess.htm).
Anyway, looking at the list of C goddessses, here are the ones that brought my attention:
Cailleach Bheur - Personifies winter (reminded me of C.C.'s snow comment), Also called "The Witch of Ben Cruachan" ('witch' comment in ep. 25). Also called 'the veiled one'.
But, this goddess is personified as a hag, which makes me think it's not our C.C.
Cerridwen - A magician/witch, her name meaning "Chiding love" and "White/blessed", she also made a potion that granted wisdom to whoever drank 3 drops, she also transformed into a hawk in one of her stories. (I dunno about you, but the Geass symbol looks like a bird to me).
Because of the "granting wisdom" thing, possibly C.C., but I doubt it.
Cliodna - Said to be goddess of love and beauty. She was said to have three brightly colored birds(bird thing again). Also, apparently she left the place where the immortals were to be with her mortal lover (reminds me of C.C.'s comment about "how my lover would..." in episode 11) but she ended up being pulled into the sea by the tide and drowning, leaving her lover desolate. But, she ended up "living on" somewhere else. (This confuses me a bit, but since Cliodna 'drowned' and then 'lived on', it reminded me of how C.C. 'dies' but then doesn't 'die') There's really not much to go off of other than the bird thing and the lover bit, so I doubt this is C.C.
Corra - A goddess of prophecy (Arguable, considering how much C.C. knew in episode 1 and her geass-forehead thing) and took the form of a crane (Geass = bird thing again) and symbolized knowledge and transitions to the Otherworld.
I dunno, the prophecy thing is kinda cool, but there's not enough info about the net on this goddess so I can judge this further.