A souterrain, an underground stone-built passage and chamber system typically associated with early medieval settlement in Ireland, had partly collapsed, and locals recalled a similar event happening in the same spot back in the 1970s. What the 2004 collapse exposed, once the opening was widened for investigation, was not a simple void but the entrance to a complex of at least four chambers running underground at varying depths.

The construction is careful and varied across its different sections. Chamber 2, the most elaborately built of the sequence, is oval in plan and lined with thirteen upright stones, each roughly 90 centimetres tall, with earth and small stones packed behind them for stability. Above these uprights, the builders used corbelling, a technique where successive courses of stone are laid so that each projects slightly inward over the one below, creating a narrowing vault that is then capped with a single large stone. A low, lintelled passage, less than half a metre wide in places, connects through to Chamber 3, which is earth-cut rather than stone-built and sits about 30 centimetres lower than the chamber preceding it. Spring water seeps or flows down the walls of Chamber 3, suggesting the structure intersects with natural groundwater at that point. Chamber 4, also earth-cut and fitted with a flat roof, lies beyond another short passage to the south-west. Blocked construction shafts, vertical openings used during building to remove spoil and then sealed once the work was complete, are visible in Chambers 3 and 4, as well as in the initial collapse area near the surface. The presence of these shafts, and the mixture of stone-built and earth-cut construction, points to a structure built in stages, possibly by different hands or at different times.