I am a Celtic enthusiast, not just from some genes but also from historical amazement.
Here's what I've learned, first off the Celts were a conquering warrior people. They were a branch of the great Indo European migration through Europe via the Russian steppes. ( Scythians, Cimmerian's, sarmatians) are the major example of these people. If you want the best authentic history of this then Google " the text of the Declaration of Arbroath" this scripture attests to the origin of Scottish Gaels. Funny enough if you read the Irish "Book of Invasions" the story of the Gaels is almost exactly the same. As we all know the Scots (Gaels) came through northern Ireland and setup the kingdom of Dal Riata in Scotland. The Gaels were a different and later branch of celts than the Britons who were already there and were most likely Celts from Gaulish tribes.
The other people that are shrouded in mystery is the Picts, who also were already there when the Scots came over. There are many theories of their origin but since the Britons were Celts who arrived at an earlier time than the Gaels, the Picts were probably a branch of the Britons that kept moving north. Also the Welsh call themselves Cymry, which could be a connotation to Cimmerian. And on that note there was also a Celtic tribe called the Cimbri which doesn't just seem like a coincidence.
The biggest evidence of where Celts have been is place names beginning with Gal. We know them from Galatia (modern turkey) Galicia (Poland) transalpine and cisapline Gaul (northern Italy) Galicia (Spain).
So all over Europe was the first encroachment on previous settlements of a more Nordic subclade. Soon to follow by germanic invasions. Let's keep this conversation going!