The site is recorded as a souterrain, an artificial underground passage or chamber of the kind typically associated with early medieval Ireland, often built for storage, refuge, or both. What sets this particular example apart is its near-total invisibility: there is no visible surface trace, and the only real evidence for its existence is a local tradition of an underground passage, written down by a researcher named Broker in 1937.

The souterrain sits within a ringfort, the circular earthwork enclosure that was the standard farmstead type across early medieval Ireland, roughly from the fifth to the twelfth century. Ringforts and souterrains frequently occur together; the underground structure would have been accessed from within the enclosure, offering a concealed space in times of trouble or a cool environment for storing dairy produce. At Ballyvouskill, the ringfort itself carries a separate record, and the souterrain is understood to belong to it. Whether Broker encountered the tradition through local informants or some earlier documentary source is not recorded, but the detail that this passage was spoken of in the 1930s suggests it lingered in local memory long after any physical access had been lost or sealed.