This question is asked the most frequently of any of the Irish descended. It's not an easy question to answer because actually nobody really knows the precise answer. Myth, theory and legend are used to define where the term came from.
The term "Black Irish" may refer to a group of Irish on the Island of Montserrat or "Emerald Isle of the Caribbean." The Irish intermarried with Africans and the term may have applied to the descendants. These folks have an Irish Accent and many speak fluent Irish.
It is also used as a "not so nice" term in regards to the Protestant Irish that support or supported Ulster Rule or "Black Ulster". The Catholics in Ireland still currently use this to describe their Protestant Irish "cousins".
Also "Black Irish" is often a term used to describe the dark hair, dark eyes, and the darker skin of certain Irish. According to these folks it's simply a language thing. Merely a way of describing people. For instance "Se�n Dubh" or "Black John" just as "Se�n Rua" would mean "Red John".
Another tale is genetics. That the Milesian invasions to Ireland produced the "Black Irish" or perhaps just the blow-ins from trading with Spain. However they got there it is a description of those Irish with Spanish blood.
Yet another version says that the children of Tuatha De Danann's daughters are the "Black Irish" because they inter-married with the Milesian's (Spanish) and everyone else has the fairer complexions.
One last version says it's actually a reference of the first inhabitants of Ireland who were supposedly dark haired and small of stature. Everyone else are merely descended from invaders and therefore are not "Real Irish".
I did find one interesting definition. That the "Black Irish" or "Black Dutch" refers to a group of Native American's who passed themselves off as "Black Irish" to avoid being labeled as "Indian".
14:19 Sat 06th Oct 2007