I believe most experts think the Proto-Celts broke away into their own separate group (possibly/probably a split with the Italics who went south) in an area around modern day Austria, making that a sort of “Celtic homeland.”
That was a 19th century hypothesis linking the ethnogenesis of the Celtic language with the Hallstatt culture but it has pretty much been decisively refuted as relying on misreadings of Greco-Roman authors and tenuous identifications between archaeological and linguistic evidence. We don't really know where the Celtic Urheimat was, but somewhere in France (centrally-located for our attested spread of Celtic languages, fits with what later Roman authors say) or northern Italy (where our earliest inscriptions are and closest to its nearest linguistic neighbour, Italic) are reasonable.
As for genetics, the last major replacement wave in western Europe, that of the bronze metallurgists from the Ukrainian steppe carrying the aforementioned R1b, happened somewhere around a thousand years before the expansion of the Celtic language (maybe). Possibly it can be identified with proto-Indo-European, but even this shouldn't be taken too far because for example modern Basque men have R1b at 87.1%, and they obviously don't speak an Indo-European language. The relationship between genetics, language and material culture in prehistorical societies is waaaaaaay too complicated and obscure for anyone to have figured out a model of how they interfaced.
