A souterrain is an underground stone-built passage or chamber, typically constructed during the early medieval period as a place of refuge, cold storage, or concealment. What makes this one notable is sheer length: the second of its two chambers runs approximately 22 metres from north to south, which is a considerable span for a structure of this type. Much of it has collapsed from its midpoint southward, and by the time it was surveyed the northern end was inaccessible, which means the full picture of the space remains incomplete.

The souterrain sits within a ringfort, the circular enclosed settlements that were the dominant form of rural habitation in early medieval Ireland, typically occupied between the sixth and twelfth centuries. The two stone-built chambers may originally have been connected, forming a single underground network beneath the fort. Chamber 1, aligned northwest to southeast and standing around 1.5 metres high, is largely infilled now but retains two air vents, small openings that would have provided ventilation and helped maintain a cool, stable temperature inside. Chamber 2, the much longer of the pair, runs roughly north to south to the west of the first. Its partial collapse, and the inaccessibility of one end, leaves the structure somewhere between ruin and enigma. The survey data comes from D. Lavelle's archaeological survey of Ballinrobe and district, published in 1994, which documented the site as part of a broader effort to record the archaeology of the Lough Mask and Lough Carra region.