Complex sentence: The ancestor of many of the succeeding rulers of Mann and the Isles he also became King of Dublin, but no contemporary source refers either to him or any of his predecessors as "King of Mann and the Isles" as such.
Split into simpler sentences: The ancestor of many of the succeeding rulers of Mann and the Isles, he also became King of Dublin. However no contemporary source refers either to him or any of his predecessors as "King of Mann and the Isles" as such.