Erraid is roughly one mile square, attended by a small archipelago of islets with poetic names: Eilean Dubh (two of them), Eilean nam Muc, Eilean Chalmain, Eilean Ghomain, Eilean na Seamair. It lies west of the Ross of Mull, separated by a beach passable on foot at low tide, and southeast of Iona across the open sound. The bedrock is granite, the same coarse pink stone that quarrymen split out of the Ross of Mull to build the great Hebridean lighthouses of the nineteenth century. Despite sitting in some of the wettest territory in Europe, Erraid receives about 1,000 millimetres of rain a year and roughly 1,350 hours of sunshine, making it one of the driest and sunniest places on the western seaboard of Scotland. A narrow but deep channel separates the island from the westernmost Eilean Dubh, useful enough that the Northern Lighthouse Board chose Erraid as a working base when they built Dhu Heartach offshore on the Torran Rocks.

Thomas Stevenson, Robert Louis's father, was one of the engineers who designed and built lighthouses for the Northern Lighthouse Board. Stones for Dhu Heartach were quarried on Erraid and fitted together on the island before being shipped to the reef where the tower was assembled. The Board built a small terrace of granite cottages on Erraid to house lighthouse keepers and their families, and the row still stands. Young Robert Louis Stevenson visited several times, recording one excursion in his autobiographical Memories and Portraits. The landscape worked its way into his fiction. The island of Aros in his short story The Merry Men is recognisably Erraid in disguise. And when he needed to maroon Davie Balfour somewhere bleak and confusing for the middle of Kidnapped, he wrecked the brig Covenant on the Torran Rocks south of Erraid and washed Davie ashore. The whole island, Stevenson wrote, is nothing but a jumble of granite rocks with heather in among.

Davie Balfour stayed alive on limpets and periwinkles. I knew indeed that shell-fish were counted good to eat, Stevenson wrote in his hero's voice, and among the rocks of the isle I found a great plenty of limpets, which at first I could scarcely strike from their places, not knowing quickness to be needful. And so hungry was I, that at first they seemed to me delicious. The hunger and the cold and the rage at his fate kept Davie from noticing the obvious tide. A sea-bred boy, he conceded, would not have stayed a day. After the lighthouse keepers left, Erraid passed through private hands, and a small intentional community linked to the Findhorn Foundation now lives in the old keepers' cottages. In September 2021, Ben Fogle visited the island for an episode of his Channel 5 series New Lives in the Wild, sitting down with permanent community member Philip Hetherington in a row of nineteenth-century granite that Stevenson's father built.