Hakan Rotmwrt

except we have no evidence of them speaking a common language

We know for a fact there's no genetic link between them. Some say there's linguistic evidence, relying on the idea that Lusitanian languages are 'Celtic' - an unverifiable claim.

I think we need to distinguish between strong & weak arguments: Strong: the iron age inhabitants of Gaul were similar enough such that they appeared to be a similar culture to their outgroup and wandered around occaisonally Weak: these pepo also lived in Ireland and the UK

That "strong" argument is in fact weak. All the sources we have about so-called "Celts" are co-dependent and mostly derive from Herodotus. I've seen every single iteration of the word 'Celt' in Greek and Latin printed on five A4 pages, 95% are based on a single use...

The Gallic invasion of Greece happened 200 years after Herodotus died...? Was Strabo just making shit up when he connected the two?

It’s weaker than the Hungarian myth of continuity with Huns. Anyway, ancient genomes reveal that a population exactly the same in their corrupted genetics as the modern Irish were in place there long before insular Celtic and even “Proto-Celtic” are supposed to have existed.