The evidence for it is a gap in the ground and a story passed down through at least two generations: a local man recalled his father going into a cave, the entrance of which lay in a hollow just outside the northern edge of the fort. That entrance has not been relocated since. What remains visible today is an oval depression roughly six metres across, running northwest to southeast through the northern half of the enclosure, which may represent a souterrain roof that has given way.

A souterrain is an underground chamber or tunnel, typically dry-stone lined, built during the early medieval period and associated with the ringforts that were the dominant settlement form in Ireland from roughly the fifth to the twelfth centuries. They served various purposes, most likely food storage, refuge, or both. The ringfort at Derryglassaun, recorded separately in the archaeological record, is the kind of enclosure within which souterrains are commonly found. Writing in 1983, Claffey noted the collapsed hollow and the local tradition of a cave but stopped short of confirmation. The entrance hollow to the north, described in the tradition as distinct from the fort itself, suggests the passage may have extended beyond the enclosure boundary, which is not unusual for the type.

What makes this particular site quietly compelling is the precision of the memory against the absence of physical evidence. The father went in; the son remembered it; the entrance was not found again. Whether the ground has shifted, whether vegetation or accumulated debris has obscured the opening, or whether the whole structure has now collapsed entirely is unknown. The oval depression is the only thing that remains legible on the surface, and even that is offered only as a possibility.