Early in the twentieth century, a passage was recorded leading into the north-east corner of an enclosure, descending into a souterrain beneath. Today, that opening can no longer be found.

A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, and most often found within or close to a ringfort or enclosed farmstead. They were used variously for storage, refuge, or as a cool environment for dairy produce. The Kilnahone example was noted by O'Leary in 1918, who described an opening at the north-east angle of the enclosure that gave access to the underground structure. Whether the passage became blocked through deliberate infilling, natural collapse, or gradual accumulation of soil and vegetation in the century since is not recorded. The enclosure itself, catalogued separately, presumably still survives in some form, but the souterrain's entrance has been lost to sight in the intervening decades.