Whether or not that tunnel still holds its shape underground, the evidence on the surface is suggestive: a depression roughly 5.6 metres wide and 12 metres long, sitting immediately west of an interior mound and travelling westward into the earthwork itself. This kind of feature is consistent with a souterrain, an underground passage or chamber built during the early medieval period, typically from stone-lined or rock-cut tunnels, and used variously for storage, refuge, or escape. The word itself comes from the French for "underground stream", though the Irish versions were entirely man-made.

The ringfort at Ballinglanna is described as univallate, meaning it has a single line of enclosure rather than the double or triple circuits found at higher-status sites. That enclosure takes the form of a high, steep bank of earth and stone with an external fosse, the term for a defensive ditch cut around the perimeter. The ground inside sits noticeably higher than the surrounding land, and the remains of what appear to be house-sites have been identified within. This kind of enclosed settlement was typical of early medieval Ireland, used by farming families as a protected homestead. The possible souterrain, if that is what the depression marks, would fit naturally into that picture: such passages were commonly incorporated into ringforts, often connecting the interior to a concealed exit beyond the bank. C. Toal documented the site in the North Kerry Archaeological Survey, published in 1995, and that survey remains the primary source for what is formally recorded here.

The tunnel's route, if it exists intact below ground, has not been excavated or confirmed. What the surface offers is the depression, the local stories, and the particular way the land dips and draws the eye westward toward the bank.