This is a souterrain, a type of underground passage or chamber built in early medieval Ireland, most commonly associated with raths, the circular earthwork enclosures that dot the Irish countryside. Their exact purpose remains debated, though storage, refuge, and cool-keeping of foodstuffs are all plausible explanations. What makes this particular example quietly compelling is the detail of its construction and the deliberate difficulty of its interior geography.

The entrance, located in the south-eastern quadrant of the rath and set 21.6 metres inside the inner bank, opens into a drystone-walled passage running roughly north-north-east to south-south-west. The passage stretches four metres in length, narrows to just 90 centimetres across, and rises from 75 centimetres at one end to 1.2 metres at the other, the ceiling formed by three flat lintels. At the north-north-east end, the passage appears to close against drystone walling. At the south-south-west end, it curves and contracts sharply into a creepway, the walls formed by two large sandstone blocks and roofed by a single stone, the whole space measuring just 34 centimetres wide and 27 centimetres high. This near-impassable constriction, slightly corbelled where the walls lean inward above, then opens into a small circular corbelled chamber, roughly 1.7 metres in diameter and approaching two metres in height, its roof sealed by a single large stone. The corbelling technique, where successive courses of stone are laid so that each projects slightly inward over the one below, allows a domed space to be enclosed without mortar or a keystone.