Complex sentence: Undoubtedly a Brythonic leader of substance established himself in north Wales, and he and his descendants defeated any remaining Irish Gaelic presence and incorporated the settlements into their domain and reoriented the whole of Gwynedd into a Romano-British and "Welsh" outlook.
Split into simpler sentences: Undoubtedly a Brythonic leader of substance established himself in north Wales. He and his descendants absorbed and incorporated Irish Gaelic settlements into their domain, and reoriented the whole of Gwynedd into a Romano-British and "Welsh" outlook.