jO_77,

I don't know of any pan-celtic calendar. The most detailed pre-Christian calendar I know of from Celtic-speaking western Europe is the Coligny calendar, a lunisolar calendar dug up from a ruined Roman or Romano-Gaulish temple of Apollo or Mars (the identity of the god is uncertain) in 1897. I suppose this was a good calendar for the temple where it was used (it's reasonable to assume it was used there, though even that isn't certain). There is no evidence it was used anywhere else. And southern Gaul is a long way from Ireland.

That the Celtic languages originated as a single language doesn't argue for a common calendar once celtic languages had been imposed on a wide area.