They Danelaugh (or Danelaw) did add Viking blood to the Irish and other British populations, but red hair was prevalent in Celtic lands before that era. Romans considered the stereotypical Germans just to their north to be red-haired and blue eyed (instead of blond haired), and a lot of those ‘Germans’ were Celtic (same with the Thracians) and while Germanic, Germans today aren’t Nordic.
Romans had a penchant for wearing german hair. Blond colored hair was considered desirable back then too, but they didn’t have the ability to dye hair back then. So what they did was hunt “barbaric” germans for their hair. They made wigs out of german hair and wore them. Romans had naturally dark hair.
I’ve never heard of these ancient celts being red haired. They were called gauls back then, or gallic. This is why the irish language is called gaelic. gallic...gaelic...same thing. They all wore plaid and marched to bagpipe music. To my knowledge, the gallic people were black haired and black bearded. The romans called them ghouls...a derrogatory term we still have today that means monster or blood thirsty demon. Gaul...ghoul...same thing.