The structure is a souterrain, an underground passage or chamber of dry-stone construction associated with early medieval ringforts, typically used for storage, refuge, or both. What makes this particular example quietly remarkable is the manner of its survival and the precision of its construction, all of it invisible from the surface and entirely unannounced by the landscape above.

The souterrain sits at the centre of what was once a ringfort, a circular earthwork enclosure common across early medieval Ireland. When the fort was being levelled in 1962, machinery accidentally punctured the roof of the chamber, revealing the structure below. It was not properly excavated until 1972, when archaeologist Twohig documented it before it was backfilled again. The chamber itself is roughly rectangular, measuring 3.6 metres long, 1.5 metres wide, and 1.4 metres high, with corbelled walls (built by laying stones so that each course projects slightly inward, creating a self-supporting curve) and a roof of flat lintels. From the north-west corner, a narrow ventilation shaft, just 30 centimetres wide and 20 centimetres high, slopes upward to the original ground surface, stone-lined and capped with flat stones. Entry would originally have been made by descending a vertical shaft some 2.2 metres below ground level, then crawling through a low creepway, barely half a metre wide, to reach the chamber. Crucially, excavation revealed that the souterrain had not been dug directly into the earth but was built as a free-standing stone structure inside a large rectangular pit, which was then backfilled around it. This is a more labour-intensive method than simple rock-cutting, and it suggests considerable deliberate effort in the original construction. The fill of the entrance shaft yielded a significant quantity of animal bones and charcoal, and a shallow trench overlying the shaft may represent the foundation of a later house built across the now-disused underground space, suggesting the souterrain fell out of use while the ringfort above it was still inhabited.