A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with ringforts, the circular enclosed settlements that were the dominant form of rural habitation in Ireland from roughly the sixth to the twelfth centuries. They served variously as storage spaces, places of refuge, or both. The one at Garryduff has not been excavated or formally confirmed; what exists instead is a slight depression just inside the western bank of the ringfort, the kind of subtle surface hollow that can indicate a collapsed or silted-up underground structure beneath.

The earliest published reference to anything unusual about this ringfort comes from a 1917 work by the Cork antiquary Patrick Power, who noted that the site was what he called 'chambered', suggesting he was aware of some subterranean feature, whether from local knowledge, surface observation, or earlier accounts. That single word, recorded over a century ago, is the most direct indication that something lies underground. Ringforts with souterrains are not uncommon across Munster, but the particular combination of a historically noted 'chambered' quality and a visible depression in the ground makes Garryduff a site where the archaeology is, in a sense, still waiting to be read properly.