Complex sentence: Although the young Dubgall may well have been the nominal King of the Isles following Guðrøðr's final defeat, the chronicle makes it clear that it was Somairle who possessed the real power, and certain Irish source regarded Somairle as king by the end of his career.
Split into simpler sentences: Although the young Dubgall may well have been the nominal monarch, the chronicle makes it clear that it was Somairle who possessed the real power. Certainly, Irish sources regarded Somairle as king by the end of his career.