This is a souterrain, the term used for an underground or semi-underground stone-built passage or chamber that appears throughout early medieval Ireland, typically associated with ringforts and believed to have served as refuges, storage spaces, or escape routes. What makes this one quietly arresting is its simplicity: a single subrectangular room, 4.8 metres long and 1.6 metres wide, closed at the back by one large upright slab and roofed with capstones laid over corbelling, where the side walls are stepped inward course by course to carry the weight above.

The structure sits within a hut site that is itself part of the broader ringfort complex, suggesting a cluster of activity that would have made up a small farmstead or defended settlement during the early medieval period. The corbelled roofing technique is a dry-stone method with deep roots in Irish and Atlantic European building tradition, requiring no mortar and relying instead on careful placement and the pressure of the stones against one another. The chamber's interior has at some point been dug into, which is a common fate for souterrains, whether by treasure seekers, later agricultural activity, or simple curiosity. The architectural details recorded here come from a 1994 survey of the Ballinrobe district, which documented the archaeology of the Lough Mask and Lough Carra region of County Mayo.