Compare Irish mythology with that of the Greeks and there is quite a bit of parallel, and a particularly bright outlook as compared with the darker Norse gods. there are also som eindications in artwork and craftsmanship that the more barbaric Norse really didn’t master for a long time.

Although the population has mixed so much it’s difficult to tell anymore, the population of Irish and Scots redheads was predominantly along the North Eastearn coastlines of the Isles.

So yes, the red hair is a sassenach legacy, from far enough back that it doesn’t matter so much any more, especially given the fairly inclusive nature of Celtic tribalism, where even children born by slaves made their way as free men.