A souterrain, to use the proper term, is a stone-lined underground passage or chamber typically associated with early medieval settlement in Ireland, most often interpreted as a place of refuge, storage, or both. The one at Ballybeg fits the broad type: stone-lined walls, an enclosed subterranean space, and what appeared to be animal bones among its contents.

What makes this particular site quietly frustrating is the speed of its disappearance. Discovered during what was presumably agricultural work in the tillage field, it was described by those who found it before anyone with archaeological training had a chance to inspect it. The bones, thought to be animal rather than human, were noted but not formally examined. Then the souterrain was filled in. The window between discovery and obliteration was narrow enough that the site passed directly from unknown to recorded-but-unexamined, leaving behind only local testimony and a map reference on the western slope of the glen. It is the kind of site that archaeology occasionally has to account for, a structure that existed, was seen, and is now gone, its fuller story inaccessible.