Red hair is a Norse trait.
It was not Celts who had red hair, it was the invading Vikings from Norway who had red and blond hair, Red hair is brown hair with less colour pigment, it usually happens when the parents, one has black hair and one has blond, hence many Red haired parents have blond children but it doesn't happen the other way round and when it does it's quite rare (unless there's red elsewhere in the family, red hair can skip generations). It is not a 'G' Celtic (Manx, Scottish or Irish) trait. Thought you should know since you have misleading information on your website. The dark haired, blue eyed, pale skinned folk are the Celts.