Ceridwen is one of those interesting figures in British myth and legend that seems to occupy both the historical and the mythical realms at first sight. She is clearly historical in her associations; she is married to a fifth-century king named Tegid Foel, a peninsula across from Welsh Anglesey named Lleyn. She is also known as the mother of the warriors Sanddef and Morfan, both associated with Arthur, who was active around 500. Finally, Ceridwen is connected with Taliesin, a figure of the mid to late sixth century.
Ceridwen also has supernatural qualities which has led to associations with gods in the past, but which are better linked with medieval conceptions of witchcraft. These present in the conception of Taliesin. Morfan was a repugnant child, so the story goes, and his mother decided that he should have wisdom and inspiration to compensate for it. To do that, she concocted a potion that would need to brew for one year and a day and from which the first three drops would grant wisdom. Then she assigned a boy to stir it. Three drops landed on the boy’s thumb, giving him wisdom when he sucked on it. Realizing what he’d done he fled Ceridwen, morphing into various guises as he did so. Ceridwen each time found him and transformed into a predator. Finally she swallowed the boy in the form of a kernel of corn. In time she gave birth to Taliesin, whom medieval legend assigned otherworldly abilities of time and space.
Unfortunately, her historical connections fall apart on further review. Medieval wives are virtually unknown in the first few centuries after Rome fell because they held no military or political significance. The few women who are associated with kings in this period (Gwenhwyfar/Guinevere comes to mind) were instead avatars of the land a king ruled, the living personification of the land. Thus if Guinevere was not an historical queen she would have been the symbolic kingdom for every ruler of that area from time immemorial till well into the historical era. Similarly Ceridwen, if she was Tegid’s queen, would not have existed as a real person.
This leaves her magical qualities, though all of them are only mentioned in conjunction with Taliesin; swallowing and conceiving reminds one of how Zeus impregnated Demeter. A cauldron of inspiration is a motif to be found extensively in Celtic as well as Norse myth.
The more one knows about Ceridwen, the less one can certainly pin on her. One thing is clear, however. There is about her no story involving a sword. She is only connected to Arthur through her two sons. And, though her husband is said to have been active around Llyn Tegid, modern Bala Lake, she is not connected with it. For all of this, she is in no way directly connected with Ninian.
However, it is possible that she was in some way an inspiration for a single aspect of the Lady of the Lake. If she was the avatar of Lleyn’s kingship, and if that kingship was associated with a specific sword at some point, it is conceivable that the spreading Arthurian corpus could have absorbed a particular scene of her giving that sword to the new king from local legend, in the process stripping it of Ceridwen and Tegid’s name and supplanting them with Arthur and the more widely known Ninian. However, that is no more than idle speculation; a guess.