True celts tend to be dark haired and dark eyed. The lighter celts are so because of mixing with anglo-saxons and danes. Ever heard of danelaw? The british isles were ruled by denmark for a long time. Vikings plundered the celtic lands just as they did the rest of europe.
“nordic” refers to a genetic type that, thanks to viking raping and pillaging, has become the predominant type of all whites. Nordic genes are not limited to scandinavia.
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.