"I've heard that the red hair, common in Ireland, is from the Norse. But, now I have to wonder."
The Celts who had come from the continent also had blonde and red hair.
The "Irish" language is gaellic or celtic; from those same Celts that came from the continent.
Dublin is said to have been founded by Vikings, chromosomes or no chromosomes.
That includes blond hair.
The greatest incidence of red hair occurs in Wales, and they had the least contact with the Scandinavians.
Remember, there are three sources of populations in Scandinavia. First there are the Sa'ami. Secondly there are the Balts (whose migrations are well known). Thirdly there are the Irish taken as slaves (or buddies) into Scandinavian society.