A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with nearby settlement, used variously for storage, refuge, or ventilation. What makes this particular case quietly compelling is how little has been confirmed: the only indication of the underground feature is a large flagstone lying to the south-west of a depression in the platform's surface, which was traditionally said to mark the entrance.

The ringfort itself, known locally as Ballinvoher Fort or An Lios, sits on level ground about 200 metres east of a tributary of the Owenascaul river, directly west of the village. A rath of this kind is a circular enclosure defined by an earthen bank and outer ditch, or fosse, the standard form of enclosed farmstead in early medieval Ireland. This one measures roughly 37.7 metres north to south and 40 metres east to west, making it a substantial example. The interior is largely empty of visible features except for that raised platform in the eastern sector, a rectangular mound measuring approximately 12 metres by 16.6 metres and half a metre high. At its centre sits a depression, around 5 metres by 4 metres, and it is beside this hollow that the flagstone lies. The association between the stone and an underground entrance was recorded in the Ordnance Survey Name Books for Ballinvoher, and was later noted in the 1986 Dingle Peninsula archaeological survey compiled by J. Cuppage for Oidhreacht Chorca Dhuibhne.

The flagstone is visible from the surface, though what lies beneath it remains unexcavated. The platform and its central depression are the things to look for once inside the enclosure, and the relationship between that hollow, the stone, and whatever passage or chamber might run beneath the platform is the question the site quietly poses without yet answering.