A souterrain, the term used for the underground stone-lined tunnels built in early medieval Ireland, typically as places of refuge or storage, once lay within the interior of a rath, the circular earthen enclosure that served as a farmstead during that same period. Today, neither feature makes much of an impression on the landscape. The rath itself survives as a recorded monument, but the souterrain beneath it has left no visible trace above ground.

The only record of the souterrain's existence comes from a note made by McCaffrey in 1952, catalogued as entry number 68 in what was evidently a systematic survey of the area. That single reference is, at this point, the entire paper trail. Whether the souterrain was excavated, collapsed, or simply absorbed into the surrounding soil over the centuries is not documented. What is known is that it sat inside the rath, which is itself the more typical arrangement: souterrains were often constructed as integral parts of rath settlements, accessible from within the enclosed area and used by the people who lived there. The pairing of the two features at Ballyculloo fits a pattern common across early medieval Ireland, even if almost nothing of it remains to be seen.