Well, they'd not be 'emerging', it'd be like reforms. The Errain were Britons who became the first Gaels under subsequent influences and new settlers. They still remained the most powerful tribe and tribute hordes imply they ruled most of the island at a point, and probably parts of continental Armorica and British Caledonia at various times. Gaels weren't a random seperate tribe, it's a culture that emerged and included within it many different tribes. If they were in at all, they'd be the Errain tribe in southern Ireland, who would start out pretty similar to midland Britons, but develop differently as the game went on.