Complex sentence: The strongest evidence has Manchan of Mohill flourishing in the 5th and 6th century, because the Annals of Tigernach confirms the scribes of Iona Abbey recorded 538AD as the year.
Split into simpler sentences: The strongest evidence has Manchan of Mohill flourishing in the 5th and 6th century. The Annals of Tigernach, which is associated to the scribes of Iona Abbey, records 538AD as the year.