A series of depressions in the interior of the fort trace out a roughly square area, approximately 10 metres by 11 metres, and the working interpretation is that they mark the collapsed or silted-over position of a souterrain beneath. A souterrain is an underground passage or chamber, typically built during the early medieval period in Ireland, usually from dry-laid stone, and used for storage, refuge, or both. What makes Dunmarklun quietly interesting is precisely this uncertainty: the earth has subsided in a way that suggests something hollow underneath, but the structure itself remains unexcavated and unconfirmed.

The ringfort with which this possible souterrain is associated is a separate recorded monument, and the pairing is entirely typical of early medieval Ireland, where souterrains were frequently constructed within or directly beneath the enclosed farmsteads that ringforts represent. The depressions at Dunmarklun were noted and catalogued as part of the archaeological inventory work covering Mid Cork, published in 1997. That survey recorded the dimensions and location but stopped short of any excavation, leaving the site in a state that is common for souterrains across the country: suspected, mapped, and left largely to the ground itself.