Supposedly, Spanish sailors from the 1588 Armada who crashed on the coast of Ireland and intermingled with the Irish population, introducing Mediterranean genes. That is the story of the 'black irish.' However, I'm not so sure if it is true or not.
That's a popular legend; but many of the shipwrecked Spanish sailors were killed by the Irish. One man in west County Clare claimed to have single-handedly killed more than 100 Spanish sailors, many of whom were ethnic Arab mercenaries from Morocco. The 'Black Irish' came many centuries earlier: Celtic tribes set out from northern Spain and Portugal and headed north to Ireland in about the 3rd Century, AD. I have done some research on this subject and concluded that they were driven out, either by an invading horde or (more likely) by a series of natural disasters. They headed north, 600 miles across the
mar Cantabrico, leading me to believe that their problem was coming from the South. If you visit some
Castreno or Celtic village sites in the north of Spain you may conclude, as I did, that they left in haste.
There was another Celtic migration around the 7th or 8th Century AD into the northern part of Ireland.
There's a place called Spanish Point in County Clare; there's an Armada Hotel there and a great 9-hole Links course.
There's a Spanish Arch in Galway and there was a Mayor of Galway named Lynch who sentenced his own son to be hanged for some indiscretion with a Spanish NobleWoman.
The earlier immigrants probably made landfall in Munster (Kerry, Cork, etc., although some may have made it to the north of the Shannon Estuary (Clare, Galway & Connemara, Mayo: the latter are in Connaught).