It is a souterrain, an underground or semi-underground stone-lined tunnel of the kind built throughout Ireland during the early medieval period, typically serving as a place of refuge, storage, or concealment. This one runs east to west for five metres, standing just over a metre high and a little under two metres wide, its walls built from dry-stacked, uncoursed limestone rubble with no mortar to hold them together, relying entirely on the weight and fit of the stone.

The roof is formed by large roughly rectangular lintels, each averaging nearly two metres in length and around forty centimetres thick, laid across the top of the passage walls. These are substantial slabs, and the care taken in selecting and placing them speaks to the effort the original builders invested in the structure. The eastern end of the passage is now blocked with collapsed earth, closing off what may once have been a second opening or the primary entrance. The western end terminates in a pointed wall, a slightly unusual detail in itself. The entrance visitors can access today is not thought to be original; it passes through a gap between the lintels toward the western end, most likely a later breach in the roof rather than the intended way in. Where exactly the builders meant people to enter and exit remains unclear, which gives the whole structure a quietly unresolved quality.