It is roughly the size of a large wardrobe, two metres by one point eight, and just over a metre deep. Fragments of dry-stone walling still line its edges. Within living memory it was an entrance to a souterrain, an underground passage or chamber built during the early medieval period, typically used for storage, refuge, or both, and cut into the earth with careful stone facing. Now the passage itself is gone, or at least inaccessible, and what remains is a rectangular depression pressed against the inner bank of the ringfort it once served.

The ringfort here is described as bivallate, meaning it was defended by two concentric earthen banks rather than one, which suggests a settlement of some status. It is one of five ringforts clustered along the western side of Drom Hill, a stretch of landscape caught between the estuary of the Owenmore river to one side and those of the Scorid and Glennahoo rivers to the other, all of them draining towards Brandon Bay. That concentration of enclosures on a single low ridge implies sustained occupation, a community making repeated use of the same defensible ground over generations. The souterrain at An Droim Thiar was recorded in J. Cuppage's 1986 archaeological survey of the Dingle Peninsula, Corca Dhuibhne, which remains one of the most thorough examinations of the area's early medieval landscape.

What is quietly strange about this site is the compressed timescale implied by the phrase "within living memory." The souterrain did not collapse in antiquity; someone alive today may have crouched inside it. The transition from functional underground chamber to featureless depression happened in recent decades, not centuries, which is a reminder of how quickly earthworks can shift from legible structure to ambiguous hollow once the ground above them is no longer maintained or watched.