First is the map of Red Hair in Europe. You're not surprised that Irish and Scots have a lot of red hair.
Next is the map of a mutation on Chromosome 16. It's almost a perfect match.
Slavery? See the patch of red hair at the SW tip of Norway?
This is certainly explained by the Vikings capturing Celts from the land of modern Ireland and Scotland a thousand years ago.
Neanderthals? Notice the outlying patch in Russia. It has no connection to the Chromosome 16 mutation map. The Udmurt tribal group in the Urals at the headwaters of the Volga are as red as the Irish but unrelated. Suspicion points to a Neanderthal link. Neanderthals have about 10% of their genome present in ours and don't have the recessive mutation on #16.
If you read the article on Red Hair genetics and mapping, you'll see there are several historical details that don't quite fit this narrative. (The Tarim mummies, ancient Thracians) Most of the content of this posting is abridged from the link.