Working in the not so wee hours in the morning, I made a number of mistakes, not counting the typos in a note above. It was Gwynedd, not Gwent, opposite Anglsey Island that was cleared of Irish at about 500 CE. I also fouled up the name of Fergus in my Scots and English histories, but we already have enough of Fergus, so I won't bother to correct it.

Fergus had been king of a small teritory in northeast Ireland with a friend Mac Erac in a big kingdom along one of his borders and a number of unfriendly kings along the rest of his borders. However, Mac Erca formed a new alliance with the Ulaind in Ireland and ceeded much of his territory adjacent to Fergus's to enemies of Fergus, so Fergus left with his two brothers and a large group of his followers.

Irish Annals give his emmigration date as 503, accordin to an English history, although the writers of my Irish and Scots histories don't seem know that.

King Arthur's correct grandfather is sorted out here: Constantines Unconfused