The dark ones who inhabited Ireland before the Celts. Seeking revenge for being driven out to sea they would attack all who dared set foot upon the isle.
They are noted for being enemies of the Tuatha de Danaan and regular struggles ensued. The battle between good and evil is indeed eternal. Myths and Legends index
Black Irish?
The Tuatha de Danaan are thought to be from the Tribe of Dan in some Irish Celtic circles. That does fit in with the Proto-Celt theory of Israelites leaving Egypt several hundred years prior to the overland Exodus.
> Myths and Legends --- Black Irish?
I really don't know. Sorry I can't be of any help here. In my Ancient Irish studies at Trinity-Dublin the subject never came up, and since I am not Irish I have never been interested enough to pursue the legend/rumour.