A souterrain, the term used for the stone-lined underground passages and chambers built in early medieval Ireland, typically as places of refuge or storage, once ran beneath this ground. Today, nothing visible marks where it sits, and yet it is recorded, mapped, and known to exist.

The only direct observation on record comes from P. J. Hartnett, who noted in 1939 that a single large roofing slab was exposed at the time, and that surface indications suggested the chamber extended in a north-easterly direction. The souterrain sits on the western side of a ringfort, a type of circular enclosure used in early medieval Ireland as a farmstead, usually defined by an earthen bank and ditch. The association between souterrains and ringforts is common across Ireland; the underground chambers were often accessed from within the enclosed space, providing concealment or a cool environment for storing food. At Glenaglogh, whatever access point or exposed stonework Hartnett observed in 1939 has since disappeared back into the landscape, leaving no visible surface trace.