What emerged was a souterrain, the name given to the kind of stone-lined underground passage or chamber built in early medieval Ireland, typically associated with nearby settlement and used for storage or, in times of danger, concealment. This one had simply been waiting, unknown, beneath a working field.

The accessible portion of the structure divides into two parts: a small chamber and a creepway leading off it. The chamber is roughly pear-shaped in plan, measuring 2.1 metres east to west and 1.6 metres north to south, with a height of 1.4 metres. Its walls are built from horizontally laid slabs and some rubble, corbelled inward as they rise, meaning the upper courses lean gradually together in the manner of a dry-stone vault, distributing the weight without mortar. Two broad lintels span the top. Above the entrance to the creepway there is a small recess cut into the wall, just 45 centimetres wide and 25 centimetres deep, whose purpose is not recorded. The creepway itself runs to the north-east for only 1.35 metres before collapse blocks further progress; it is roofed with flat lintels and averages half a metre wide and 35 centimetres high, dimensions that make the name accurate. Entry to the whole structure is now made through a gap left by the removal of one of the chamber's roofing slabs during the original discovery.