There is no single ancient, verifiable Celtic creation story, i.e. a myth that has come down to us from ancient times and found in the literatures. Many modern authors have created or reconstructed such things, but these creations are not ancient and, despite their literary merit, we can’t really make claims for them representing what the ancient druids or Celtic-speaking peoples may have believed. Furthermore, seeing as it is generally agreed that some form of reincarnation and the cyclical nature of existence were probably very ancient beliefs among the Celtic peoples, i.e. a non-linear view of being, then, perhaps, there wasn’t the need for one. Finally, the ancient druids occupied themselves with the natural sciences, so if druids had managed to survive until today in an unbroken tradition, perhaps they would have been particle physicists too.
As I’ve said before elsewhere, no one can say for sure that the druids of old did not have a creation myth, but we just haven’t inherited one – absence of evidence is not evidence of absence, but by the same token, when something just isn’t there, we can’t make claims for it either.