I believe that some have speculated this, but again the Tuatha Dé are concerned with the taking of Ireland, I think it would be hard to find a world creation myth in there. It seems a lot of Celtic mythology was very local, the Irish otherworld is a very Irish one, the Welsh otherworld is a very Welsh one and so on. We don’t know what the other historical Celtic peoples might have believed, but it is perhaps reasonable to think that they might have had their own “local” mythologies. There is a Cornish droll (folk story) about how Helston got its name, involving a big fight between St Michael, the devil, a dragon and a fiery stone that crashes into the ground and becomes the “Hell-stone”. Now this relies also on a play on words in English and not ancient Cornish; nevertheless, there could be something in there in terms of an ancient tale that has been reworked and reworked through time, but again, it’s very local. I may be wrong, there may well be a Celtic version of what we would consider a creation myth and we may discover something in the future, but I wouldn’t count on it.