Coeffin Castle was built in the XIII century, probably by the MacDougalls of Lorn. Lismore was an important site within their lordship, being the location of St. Moluag’s Cathedral, seat of the Bishop of Argyll. The first written evidence of the castle occurs in 1469–70, when it was granted to Sir Colin Campbell of Glenorchy by Colin Campbell, 1st Earl of Argyll.
The ruins comprise an oblong hall-house and an irregularly shaped bailey. The great hall is an irregular rectangle, measuring 67 by 34 ft. The walls are from 6 ft 10 in to 7 ft 10 in. thick. The bailey was for the most part built at a later date than the hall. An external stair probably linked the entrance, in the north-east wall, to the bailey. A second door gave access to the sea to the south-west.