I thought the black hair came from the Spanish on the coast. Isn't that where the term "black Irish" came from?
Dunno where that term comes from, although I've heard it plenty of times throughout my life. The Normans invaded England, with William I usurping the throne (and making up a ludicrous story about how the king he killed had once told him, "if I am ever given the throne, I'll turn it over to you"). Within a century the Normans had carved out a fief on the coast of Ireland, and over the coming generations conquered and occupied the entire island. I'd make a wild guess that the black hair comes from that. However, "The Book of Invasions" details something like four (or more) semi-historical invasions by various ethnic groups, so it could have come from anywhere. :') Well, anywhere except Iberia, since it was Iberia that was invaded by the Celts, not the Celts who were invaded by Iberia. (':