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. (':
http://www.straightdope.com/classics/a930730.html
On the black Irish