The wildest notion is that black hair is evidence of Spaniards marooned in Ireland following the wreck of the Armada. As we've had occasion to discuss in the past, the number of shipwrecked Spanish sailors who remained in Ireland for any length of time was trivial.
I have also heard it said the black Irish were the first settlers of Ireland--maybe the Phoenicians. The red Irish, meanwhile, were descendants of the Normans, and the blond Irish are descended from the Vikings. One of many drawbacks to this theory is that it seems to leave the Gaels completely out of the picture.
A more plausible but still essentially unprovable take on this idea is that black hair is a vestige of an indigenous population of short dark-haired types overrun by the fair-haired Gaels. Supposedly there are more black Irish in the western part of the country, which fewer Gaelic invaders reached.
There is archaeological and, I'm told, linguistic evidence of pre-Gaelic settlement. But how it was concluded that they were short and black-haired I do not know. Seems like a silly thing to make a fuss over in any case
Yes, but it is a little more complex.
The Normans were of course Scandinavians. Rollo, their leader and the Father of William was Norwegian, and many/most of his friends were Danes. They brought Scandinavian women with them when they occupied northern France, and married local French women as well.
The Normans who invaded England from the South were Scandinavian Vikings, both Dansk and Norsk. But the large numbers of Vikings who populated York, in the NE of England, were primarily Norse. Thus, the Norman Conquest was essentially a continuation of the larger battle between Vikings for the rulership of England.
The Old Testament refers to David as "ruddy and fair". This sounds like a Red-headed Irishman. Other Israelites in the OT are also referred to as "Blond and fair", etc. This aligns with the history of Ireland.