I think everyone here is missing the boat by focusing on the concept of race or colour.
You may be interested to read about the “Black Donnellys” in Canada, a family of Irish immigrants who were massacred by mostly Irish vigilantes in 1880. See their web site at http://www.donnellys.com/This definition of “black Irish” may be only a Canadian thing but I believe it applies elsewhere in the Americas.
If you read the story of the “black” Donnellys of Canada and look at the photos, keeping in mind that early photos could be overexposed, you will note that they were all blue-eyed and fair-skinned. Their hair does not seem to have been jet-black and I see no trace of African ancestry in their features.
I believe the term refers to a lower-class Irish subcaste among the immigrant populations, who were hated, shunned and set apart by everyone, including more sucessful Irish immigrants. Like many such shunned groups, they developed a hostile attitude to society and outsiders. Poverty and social isolation ensured that they continued to be considered “black” because they would frequently be in trouble with the law, or just as frequently would be falsely accused of crimes just because of who they were.
This would perhaps explain why the Irish in Ireland are not so familir with the term “Black Irish”, since it was a phenomenon among immigrant Irish.
As little as 10 months ago I had a couple of brothers, blue-eyed, fair-skinned teenagers with medium-brown hair, from rurual Ontario, tell me that “We Black Irish stick together.”
I am sorry I did not ask them what they meant by that term, but I do know that these two came from a poor, alcoholic and socially isolated milieu.