Complex sentence: The diocese traces its history to St Patrick in the 5th century, who founded the see; it is headed by his successor, the present-day Archbishop of Armagh, who has his seat in St Patrick's Cathedral in Armagh.
Split into simpler sentences: The diocese traces its history to St Patrick in the 5th century, who founded the see. The current Archbishop of Armagh has his seat in St Patrick's Cathedral in Armagh.