The structure is not a natural cave but a souterrain, a type of underground chamber built by hand from dry-laid stone, without mortar, and used in early medieval Ireland for purposes still debated: storage, refuge, or both. This particular example has partially collapsed over the centuries and is now rubble-filled, but its basic dimensions are still recorded. It runs roughly north-northwest to south-southeast and measures 8.3 metres in length and 1.4 metres in width, which is narrow enough to make the darkness inside feel deliberate.

The souterrain sits within the northern half of a rath, the Irish term for a ringfort, a class of enclosed farmstead that was the dominant settlement type in early medieval Ireland, typically dating from around the sixth to the twelfth century. Thousands survive across the country in varying states of repair, and a significant proportion contain souterrains built into or beneath their banks and interiors. The combination here at Derrydonnell More is not unusual in itself, but the survival of a measurable, structurally distinct chamber within what is otherwise a heavily disturbed monument gives it a particular archaeological weight. The site was documented by Cody in 1989.