The discovery came when the machinery exposed the lintel stones spanning the roof, and only then did the full structure below become apparent.

What surveyors Lane and Byrne found was a souterrain, an artificially constructed underground passage or chamber, typically built during the early medieval period in Ireland and used variously for storage, refuge, or both. This particular example consists of a roughly circular stone-built chamber measuring approximately 2.74 metres by 2.54 metres, with a height of 1.7 metres, large enough to stand inside with a slight stoop. The roof was originally corbelled, meaning the stone walls were built with each successive course projecting inward until the gap at the top was narrow enough to be bridged by flat lintels. Most of those lintels have since collapsed and now lie on the chamber floor, partially buried under fallen earth. At the northeast side, a low lintelled creepway, only 0.6 metres wide and 0.82 metres high, leads away from the main chamber but is blocked by earth after at least a metre. It almost certainly connects to a second chamber or passage that remains unexcavated. A scatter of animal bones was found near the creepway and along the western side of the chamber, a detail that hints at past use without yet explaining it.

The site sits within the Carrignagroghera townland on the northern fringes of Fermoy, now absorbed into a modern housing development. The chamber, once opened in 2001, was recorded and its dimensions carefully documented, but its full extent remains unknown while that creepway stays blocked.