This is a souterrain, a type of man-made underground structure built during the early medieval period in Ireland, typically of dry-stone construction and covered with large flat lintels. They are found in association with ringforts and cashels, the latter being a stone-walled enclosure, and are thought to have served as places of refuge, cool storage, or both. The Ballynahown example opens to the north-north-west at a height of 0.8 metres and a width of 0.6 metres, dimensions that make it inaccessible to modern investigation. A stony hollow measuring roughly four metres east to west, two metres wide, and 1.4 metres deep sits nearby and is likely associated with the structure, as is a raised stony area to the west and north-west of the opening.

The souterrain's history of record is itself a little tangled. Writing in 1980, Cunningham described a souterrain "in the valley at the south face of the cliff" but incorrectly attributed it to a separate cashel, known as Cahernagrian, which lies approximately 52 metres to the north-north-west. The confusion is understandable given the density of early medieval enclosures in this part of Clare, but the souterrain belongs to a different cashel entirely. That misattribution sat uncorrected for decades, a small illustration of how closely related monuments in the landscape can blur together in the written record when the ground itself is difficult to read beneath dense scrub.