I had another look at the tatan in question, it is a Black Watch. As to the "Black Irish" term, I understood it to refer to people in Ireland who had ancestors who were survivors of the Spanish Armada, the black refers to the fact that they had darker hair and brown eyes.
You find all sorts of imaginative "explanations" for such things, usually overlooking the most obvious possibilities. The fact is that celts are typically dark haired, with a secondary blond strain showing up now and again. The redhaired Irish and Scots are usually considered to be representative of the large Viking contribution to the national gene pools. In the simplest explanation, and probably the oldest one, the Irish who come closest to the celtic type are "Black Irish" and those closest to the Viking one are "Red Irish". (This is an entirely separate concept from "Green" and "Orange" Irish.)
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