There was at least 3 major waves into Western Europe spanning thousands of years. It's not something easy to simplify. North Ireland/Scotland are fascinating, so are the Welsh peoples.
The farthest back I think I can trace Celtic is Dacians on the Danube which probubly are
"Getae or Thracians of Dacian race". And that's only "theory" which I cannot prove yet.
The human remains found in Peștera cu Oase ("The Cave with Bones"), radiocarbon dated
as being from circa 40,000 years ago, represent the oldest known Homo sapiens in Europe.