A souterrain is an underground stone-lined tunnel, typically dry-built without mortar, constructed during the early medieval period as a place of refuge, storage, or concealment. What survives at Coolnaskeagh is not the tunnel itself but its ghost: a shallow depression in the ground roughly six metres long, a metre wide, and half a metre deep, sitting just to the east of the centre of a ringfort.

The depression was recorded in a field report dated 5 July 1977, noted in relation to the adjacent ringfort. A ringfort, sometimes called a rath, is a circular enclosure defined by one or more earthen banks and ditches, and it was the standard farmstead form across Ireland during the early medieval period, roughly the fifth to the twelfth centuries. Souterrains are frequently found within or immediately beside ringforts, suggesting they served the same farming households, possibly as cool storage chambers for dairy produce or as bolt-holes during raids. At Coolnaskeagh, the ground-level evidence is tentative; the 1977 report notes only that the depression "may represent" the remains of such a structure, meaning no excavation had confirmed its nature or extent at the time of recording.