My father's parents came to the US from County Mayo. The explanation I've always heard from that side of my family is that the "Black Irish" -- black-haired, with eyes and complexions that may or may not be darker than those of other Irish -- were descendants of Spanish sailors who found refuge and sympathy in Irish port cities during various conflicts between Catholic Spain and Protestant England, especially during the 16th & 17th centuries.

I find this theory pretty persuasive. It may have been my imagination, but during my recent first-time-ever, only-one-week-long visit to Ireland, I'm sure that I noticed more locals conforming to this physical description in Galway city (an important port in post-Reformation times) than anywhere in the nearby, more rural, areas of the West of Ireland that we visited. I was especially taken back to observe many of these semi-Mediterranean-looking types speaking Irish!

I'd be willing to bet that there are also plenty of Spanish/Irish-looking natives of Cork.

Of course, the same two words "Black Irish" can also be logically applied to people of part-Irish/part-African ancestry, or to Irish residents/citizens of any African or part-African genetic heritage. There are probably many more such folks than many would imagine.