It is easy to walk past the grass-covered mound above without suspecting that anything lies beneath it, yet the earthwork conceals a souterrain of quietly precise construction.

A souterrain is an underground chamber or passage, typically built during the early medieval period in Ireland, often associated with ringforts and used for storage, refuge, or both. The example at Ballynabucky sits within the northern sector of a rath, the Irish term for a circular earthen ringfort, and was recorded by McCaffrey in 1952. The chamber runs roughly north-north-east to south-south-west, measures 5.8 metres in length and 1.3 metres across at its widest point, and was constructed using drystone walling, meaning the stones were carefully laid without any binding material. Its height varies between 0.85 and 1.45 metres, low enough to require crouching along much of its length. Entry is from the south-south-west end, and the passage tapers noticeably as it continues toward the north-north-east, narrowing to just 0.9 metres. That tapering is a feature seen in other souterrains across Ireland and may have served a defensive or concealment purpose, making rapid movement through the far end considerably more difficult.