The total land area amounts to approximately 50 hectares (125 acres) and the highest point is 88 metres (289 ft) above sea level on Eilean Mòr.

After the last retreat of the ice circa 20,000 years BP, sea levels were as much as 122 metres (400 ft) lower than at present and it is likely that the existing islands were part of a much larger land mass, although still separated from the Outer Hebrides by many miles of open water.

It is likely that the honoree was either the 7th-century Abbot of Killaloe, County Clare, Ireland, or alternatively, the half brother of the eighth century St Ronan, who gave his name to the nearby island of North Rona.

[13] The isles provide nesting for a population of seabirds, including Atlantic puffins, northern fulmars, European storm-petrels, Leach's petrels, common shag, and black-legged kittiwakes.

[15] The archipelago has been designated an Important Bird Area (IBA) by BirdLife International because it supports breeding populations of several species of seabirds.

The Scottish-American author's "atmospheric, Lovecraftian thriller" was reissued by Charles G. Waugh with an alternate ending based on an older story by Sneddon in the Lighthouse Horrors anthology in 1993.

Reverse: 1999, a turn-based tactical role-playing game, features a side story that takes place in the Flannan Isles and is based on the lighthouse keepers' disappearances.