from wiki,
The term Black Irish is an ambiguous term used mainly outside of Ireland. People in different times and places have used it for separate reasons. Opinions also vary in regard to what is perceived as the usual physical characteristics of the so-called black Irish: e.g., dark hair, brown eyes, and medium skin tone; or dark hair, blue or green eyes, and fair skin tone.
not sure that tell you anything, but there is a lot more.
My understanding is that it harks back to when the Spanish were regularly shipwrecked on the Irish Coast. Those dishy Spaniards were popular with the local lasses and babies with dark hair duly arrived...and quite a few of them.