Iona is an island in Scotland.
After the fall of Logres and Camelot, Sir Galahad took the Holy Grail to the monastery in Iona where it was kept for around three centuries until it was taken in a ninth century Viking raid.
A fragment written by a survivor of the attack, written in Old Irish (but with Anglo-Saxon influence) found its way to an abbey at Cantanez in Brittany. It formed part of the research conducted by Henry Jones to find uncover Grail's whereabouts.
The monastery mentioned in the Grail Diary is not named, but it is possible that it refers to the Iona Abbey, which was sacked by Vikings in 806 and 825.