My understanding of the Black Irish/Scots (the Scots were just Irish who left for Caledonia about the time the Romans were leaving Britain*) is that the line of descent came from the eastern Mediterranean. The same people who were ancestors to the Phoenicians had migrated across North Africa and left behind Carthaginians, came up through Spain leaving folks there and going on to Ireland. These were not, as I remember, Celts but were the people who lived in Ireland when the Celts moved in their own selves. That's the way I have it tucked away in my tiny little brain.
*This may be one of the reasons the Picts started moving in on civilized Britain causing the Britons, who had lost their Roman protectors, to call on the Angles, Saxons and Jutes for help, thus setting up the circumstances that left us with this lovely, complex, bastard of a language that has brought us all together in this little haven we call AWADtalk
