My own feeling from the Welsh and Irish texts, inasmuch as we can rely on their being a representation of Celtic thought, is that we’re dealing more with the idea of parallel dimensions: the otherworld(s) exist “next” to the physical/material world; there’s doesn’t seem to be a before or after. In the folklore traditions, numerous examples reveal a notion that one day in “fairy land” can be equal to many years in our world – again, this seems very inter-dimensional.
Another thing that has struck me is that there does not seem to be any creation myth that has come down to us from the Celtic mythological realms. Far be it from me to say unequivocally that there was none, but we certainly don’t seem to have “inherited” one. Perhaps, there was no Celtic creation myth because ancient Celtic thought did not see things in terms of beginnings and endings but rather as cycles and, as we know, a circle has no beginning nor ending. Now, I’ll add here that I have come across some stories that purport to be Celtic creation myths, however, on investigation, there’s not much to back up their authenticity and they seem to have been extrapolated by modern thinkers.