A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with raths, the circular earthen enclosures that served as farmsteads across Ireland from roughly the fifth to the twelfth centuries. They were used for storage, shelter, or refuge. The one at Corroy, if it survives intact, has so far resisted easy location.

In 1953, a researcher named R.B. Aldridge noted a local tradition placing the souterrain on the northern side of the rath. That account remained unpublished, held in the National Museum of Ireland's Topographical Files. Later survey work on the ground found something potentially more revealing on the opposite side of the enclosure: a subcircular depression roughly three to four metres in diameter and about half a metre deep, sitting close to the inner edge of the bank at the south-west. Immediately to the east of this lies another very slight depression, measuring approximately four metres east to west and three metres north to south, with a shallow extension reaching north-east. These surface irregularities are consistent with the kind of ground disturbance a collapsed or partially filled souterrain can leave behind, but their position does not match the northern tradition Aldridge recorded. Whether local memory preserved a genuine recollection of the passage's entrance, or whether the depressions at the south-west represent a separate feature entirely, remains unresolved.