Lismore (lĭzˈmôr, lĭzmôrˈ) [key], island, 91/2 mi (15.3 km) long and 11/2 mi (2.4 km) wide, Argyll and Bute, W Scotland, in Loch Linnhe. There are ruins of several old castles, one of which was a 9th-century Viking fortress, another the residence of the bishops of Argyll. The present parish church was the choir of a 13th-century cathedral. The 16th-century Book of the Dean of Lismore is a volume of Scottish Gaelic and other verse, compiled by Dean James Macgregor and his brother Duncan. It is one of the oldest Scottish Gaelic collections.