The depression, roughly six metres east to west and two metres across, sits inside the northern bank of a ringfort, and its modest depth of around sixty centimetres suggests a roof that has long since given way. What lies beneath, or what remains of it, is the probable remnant of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically serving a nearby settlement as storage space, a refuge, or both.

The ringfort with which this souterrain is associated was a common form of rural enclosure in early medieval Ireland, usually consisting of a circular bank and ditch surrounding a homestead. Souterrains were frequently constructed within such enclosures, their entrances sometimes concealed within the bank itself. At Dromkeen, the precise relationship between the underground structure and the fort above it is now largely a matter of tradition and inference, since the chamber appears to have collapsed, leaving behind only the tell-tale rectangular sag in the earthwork. That the local tradition of a souterrain persisted long enough to be recorded at all suggests the feature was at some point better understood, if not actually visible, to people in the area.