According to local knowledge, an opening once visible in the north-eastern quadrant of a rath led down into a corbelled tunnel, its sides built up with carefully placed stones and its roof laid with flat flags. A pile of logs now covers that opening. There are no visible remains above ground.

The structure is a souterrain, a type of man-made underground chamber or passage commonly associated with raths, the circular earthwork enclosures that served as defended farmsteads during the early medieval period in Ireland, roughly between the fifth and twelfth centuries. Souterrains were typically used for storage, shelter, or refuge, and their corbelled construction, where stones are laid in progressively overlapping courses to form a roof or wall without mortar, required considerable skill. The rath at Coolies in which this one sits is a recorded monument in its own right, and the souterrain is understood to lie within it, though precisely what condition the underground passage is in today is unknown. The logs covering the entrance suggest the opening has been acknowledged locally rather than forgotten entirely, even if it has not been formally investigated.