This is a souterrain, an underground chamber or tunnel constructed during the early medieval period, typically by cutting into the ground and lining the walls with carefully placed stone before covering the whole thing over. They were built for storage, refuge, or both, and hundreds survive across Ireland, though many go unnoticed even by those walking directly above them.

This particular example sits on the eastern side of the interior of Corwillick rath, an earthen ringfort of the kind that once served as a farmstead enclosure for an early medieval family. The souterrain's passage stretches 7.5 metres in length and is 1.4 metres wide, though at only 0.8 metres in depth it would have required anyone entering to crouch low. The walls are lined with drystone random rubble walling, meaning the stones were laid without mortar, fitted together by hand in an irregular arrangement rather than cut to uniform shapes. Both ends of the passage are now blocked with rubble stone, leaving the interior inaccessible and the full extent of the structure a matter of some uncertainty.