A souterrain is an underground stone-lined tunnel, typically constructed during the early medieval period in Ireland, and used for storage, refuge, or both. This one sits inside a small quarried-out area roughly six metres by eight metres, with the ground dropping about eighty centimetres to expose the structure beneath. The lintel slab alone measures two and a half metres in length, a metre wide, and a quarter of a metre thick; substantial stonework by any measure, and a reminder that whatever lies below was built to last.

The souterrain has two entrances, both of which give the impression of deliberate concealment rather than invitation. The first faces south and the second faces east, positioned beneath the western end of the exposed lintel. Each opening is 1.2 metres wide, but their heights, at twenty centimetres and fifteen centimetres respectively, are far too restricted to allow a person through. This is not unusual in souterrain design; low or deliberately awkward entrances were a common feature, slowing any intruder and giving defenders or occupants a practical advantage. The rath within which the souterrain sits, a circular earthwork enclosure of the kind used as a farmstead or defended settlement throughout early medieval Ireland, would have provided the outer layer of that same logic, combining above-ground enclosure with below-ground refuge in a single integrated system.

The site sits to the south of the centre of the Ardnabrone rath, and while the quarried area has exposed the lintel clearly enough to record its dimensions, the passage itself remains sealed and unexamined. What the underground chamber or corridor actually contains, and how far it extends, remains unknown.