This is a souterrain, an underground stone-lined tunnel built in early medieval Ireland, most commonly as a place of refuge or cool storage connected to a settlement above ground. What makes this particular example quietly compelling is less its size than the way it was built to deceive: the entrance, at the north-north-east end, turns sharply at a right angle and extends a further two metres eastward before the floor simply rises and the passage disappears. Anyone unfamiliar with the layout would have struggled to find their way in.

The souterrain sits within the enclosure of Magheraghanrush cashel, a cashel being a stone-walled ringfort of the kind built by farming families of middling status throughout early medieval Ireland. The underground passage runs roughly five and a half metres on a north-north-east to south-south-west axis, its walls built from un-coursed rough rubble limestone, the stones stacked without mortar in the drystone tradition. The original roof lintels are gone, with one exception: at the south-south-west end of the passage, where a small rectangular chamber branches off to the west, a single limestone slab still spans the top of the chamber's western wall. The chamber itself measures roughly two metres by one and a half, just large enough for a person or two to crouch inside. The floor throughout is rock and earth, sloping gently downward as the passage runs south-west, giving the whole structure a slight sense of descent into the hillside.