Two shallow hollows on the southern side of an ancient ringfort at Knockanaddoge are the only visible sign of what was once a souterrain, an underground stone-lined tunnel typically built during the early medieval period and associated with nearby settlements or fortified enclosures. Such passages were used variously for storage, refuge, or concealment, and their presence within ringforts is well documented across Ireland. Here, the surface has collapsed inward, leaving the hollows as a kind of negative impression of the structure below.

The ringfort itself still occupies the site, and the souterrain was evidently part of its original complex. Writing in 1905, the historian William Carrigan noted in his history of the diocese of Ossory that a passage leading from the fort to the west had been discovered not many years before his writing. That phrasing places the discovery somewhere in the later nineteenth century, though the exact circumstances are not recorded. Whether the passage was excavated, stumbled upon, or simply exposed by agricultural work, Carrigan does not say. What his account confirms is that the underground element was real and ran westward, meaning the two hollows visible today are likely the remnant of that same tunnel, now mostly fallen in. The site sits between the Deen and upper Dinin valleys, with open views in most directions, the rising ground to the east being the only point where the landscape closes in.