There is no evidence that red hair was introduced to to Scotland and Ireland by the Vikings.
The evidence is irrefutable, (they certainly didn't acquire the red hair from the Spanish). Why do you suppose so many place-names in Scotland and Ireland are of Scandinavian origin? Of course many have since angelicized the names, such as Dingieshowe, Birsa, Dublin and Wexford.
It is well known that the Vikings raided Ireland and established settlements. What is not established is that the
red hair came from the Vikings. If you have links to studies saying otherwise, please post them.
I'm not a geneticist but am capable of simple elimination.
Since you're so sure it wasn't northern Europeans who are responsible then who is?