What lies beneath is a souterrain, an artificially constructed underground passage or chamber, typically built during the early medieval period and associated with nearby settlements or fortifications. This one sits on the flat ground above a castle, close to the remains of an ancient fort that once occupied the far end of the same ridge.

Writing in 1904, the historian Knox noted that a couple of stones had collapsed into the cave of an old fort at this spot, and he connected the site to the O'Caidhins, a Gaelic family whose presence in the area is preserved in the placename itself. Ballykine derives from the Irish Baile O'Caidhin, meaning the townland or settlement of the O'Caidhins. Knox identified the fort as likely the cashel or caher of this family, a cashel being a stone-walled enclosure of the kind used in early medieval Ireland to demarcate a settlement or territory. The souterrain would have been a feature of that enclosure, perhaps used for storage, refuge, or both, and its partial collapse over the centuries has left the entrance exposed beneath the ridge.