Carlos is right. The "Black Irish" are nothing more than the original Celtic settlers. Red or blonde hair with green eyes was a later import, possibly from the Viking invaders. I've only been to Ireland once, but I could clearly pick out these two types on the streets of Dublin.

Here's a bit of the old Celtic story of Deirdre...(which I'm sure predates the Spanish Armada!)...

One day, Deirdre and her nurse were walking in the woods. It was wintertime, and they came upon Deirdre's foster father, who was butchering a calf. The red blood ran in the white snow, and a raven came to peck at it.
Deirdre said to her nurse,"I could love a man with those three colors: hair black as the raven's wing, skin white as the snow, cheeks as red as blood". Her nurse replied, "I know of such a man, and he lives nearby...."

There you have it, the description of the typical early Celt...black hair, and white skin with a high color.

Hope this helps,
Wyllow