St Donagh's story is one of those that could only happen in the far less bureaucratic early middle ages. Donagh was an Irish pilgrim on his way home from a pilgrimage to Rome. He wandered into the Cathedral of Fiesole just as the people had come together to pray for a new bishop. The bells rang and the candles lit of their own accord when he walked in. The people of Fiesole, being able to take a hint, decided they'd found their man and, hey presto! Donagh the Irish pilgrim found himself the new Bishop of Fiesole. And his friend and travelling companion, Andrew the Scot, became his new archdeacon.
It wasn't a sinecure by any means. Mrs D'Arcy reports that "six years earlier the feudal barons had disposed of a predecessor by drowning." And the Mahometans were making periodic raids and the bishop had defense of the diocese as one of his duties.
Wikipedia has more on St Donagh (or Donat or Donatus) here
and the Catholic Encyclopædia has something here
. But the most extensive I've seen is in Mrs D'Arcy's "The Saints of Ireland" which, alas, isn't on line. You'll need to locate your own copy.