What is a "Black Irish"?

I know it doesn't refer to an Irish person who has African ancestry. I sometimes hear this term thrown around. Some people say they are Irish people of Spanish ancestry. Some say it refers to Irish people with black hair with brown eyes (in contrast to the Celtic looking red-hair, freckles, and green eyes). Which is it? I read somewhere that Irish people (and British people in general) are mostly descended from ancient peoples from Iberia (modern Spain) before the Indo-European Celtics and Anglo-Saxons arrived.