A recent archaeology article referenced in Science News noted that Stonehenge, properly viewed, is a diagram of the female reproductive apparatus. No doubt whoever built that temple worshipped the great goddess. Was it "Scota"?
That probably cannot be answered, but it was the "Three Brothers" who sailed from the Dead Coast in Galicia to Ireland to conquer the locals, take all the women, and roast a few cows here and there. I presume they arrived with iron weapons too!
BTW, students of the Celtic past who examine it from the vantagepoint of Galicia invariably note that the coast of the Bay of Biscay has been pretty nearly dominated by Celts of one kind or the other for many thousands of years. It has also seen Celts move from one part to another and back again. Some of the Milesians moved from Galicia to Ireland circa 500BC. The same folks moved to Great Britain somewhat later to become the people the Romans met. In the early part of the Dark Ages the very same people moved on to Brittany, and as the Angles and Saxons moved in, the Celts further South in Cornwall moved back to Galicia, this time as the founders of the kingdoms that took the entire peninsula away from the Arabs.
It's not like these guys stayed in one place all the time.