Easy. The Celts were a very large group of people who modern archeology has identified as originating in region of the Cacasus Mountains as early as 600 BC.
For some reason, probably to escape enemies and to find food, they swarmed westward into Turkey and around the Black Sea, both South and North sides, and using all possible routes including what is today the Ukraine they migrated westward to the sea. They dominated what few people were in their way and already in "Europe", and avoided those they could not dominate, and eventually became the major ethnic group in Europe.
Their numbers were very large and they lived in many tribes. They fought with each other as they would with enemies, and most of the so-called "European Wars" of history were between different Celtic tribes simply carrying different names.
Does that help?
Modern archealogy has them in England and Austria by at least 1200 BC.
Yeah, that's good but too general to be of much help in deciphering prophecy. You're calling everone that migrated Celts. I think there were 12 tribes, the Assyrians, plus the original Isralite colonies. And as far as the migrations go, I think there may have been about 5 of them. 4 Israelite, and 1 Assyrian, not including the colonizing that Dan made possible. I agree with you, but there is just too many peoples that are being called celts to use the term celts.