But beneath the uneven interior, in the north-west quadrant, lies a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period, used variously for storage, refuge, or both. A large stone near the opening may be a displaced capstone, part of the original roofing structure, and three further large stones arranged side by side lie close to the south, suggesting that more of the subterranean architecture remains just below the surface.

The earthwork itself is a rath, a type of ringfort defined by a circular earthen bank enclosing a raised interior, and one of the most common monument types in the Irish landscape. This particular example measures roughly 29 metres across on a north-south axis, with a bank about 2.6 metres wide, still standing around a metre high on its outer face despite considerable erosion along the north-north-east arc. A small circular mound sits in the north-east quadrant, and a linear feature running roughly north to south near the possible souterrain entrance hints at further buried elements not yet fully understood. What anchors the site historically is its appearance in the Ordnance Survey Name Books of the 1840s, where it was noted as lying at the north-east end of Scarteen and described as containing a "cavern", that old word for the souterrain, suggesting local knowledge of the underground element had persisted long after the site fell out of use.