It is barely forty centimetres deep, between one and two metres wide, and covered over with grass. There is nothing obviously dramatic about it. But the shape it follows, turning from north-north-west to north-east across the western half of the enclosure, suggests it may be the collapsed roof of a souterrain, an underground passage or chamber typically built during the early medieval period, used variously for storage, refuge, or concealment.

The depression sits within a rath, the circular earthen enclosure that served as a farmstead or dwelling place for much of early medieval Ireland. Raths are common across the Irish landscape, but the structures associated with them are often invisible at ground level. A souterrain would originally have been a roofed, stone-lined tunnel, running beneath the rath interior; over centuries, as the stonework shifted or the timbers rotted, the ground above could subside, leaving exactly the kind of linear hollow visible here. The feature extends roughly five metres in one direction before turning and continuing for a further eight metres or so through the north-western quadrant of the enclosure, which is consistent with the branched or angled plans sometimes found in verified souterrains elsewhere in Ireland. No structural stonework is currently visible at the surface.