A roughly rectangular area of ground, measuring about four metres north to south and less than two metres wide, collapsed in the recent past, leaving a scatter of moss-covered stone slabs tilted at odd angles, some leaning inwards, some fallen entirely. The largest of them, nearly two metres long and just eight centimetres thick, is a roof lintel, the kind of flat capstone that would once have sealed the tunnel from above. It now slopes downward toward the south, resting on the stones that were meant to hold it up.

The structure beneath is a souterrain, an underground stone-built passage or chamber of the kind constructed throughout Ireland during the early medieval period, roughly between the seventh and twelfth centuries. They were typically dug into the earth and lined with drystone walling, then roofed with large flat slabs. Their precise function is still debated, but most were likely used for storage or as places of refuge, often associated with nearby ringforts or farmsteads. In Derryfadda the collapse has exposed what was previously invisible, turning a sealed underground feature into something you can look at directly, albeit in a state of considerable disorder. The site sits on a slightly raised area on a gentle north-facing slope, which is itself a fairly typical choice for a structure designed to stay cool and dry.