Whether it survives intact, whether it was ever fully passable, or whether it belongs to the class of underground passages known as souterrains, long stone-lined tunnels built within and around Irish ringforts, possibly for storage or refuge, remains unresolved. What is certain is that a depression some 12 metres long and 5.6 metres wide runs westward from a low sub-circular mound inside the fort and disappears into the bank, which is exactly where such a tunnel would need to go.

Ballinglanna Fort is a univallate ringfort, meaning it has a single line of enclosure, in this case a high, steep bank of earth and stone with an exterior fosse, a ditch running along the outside of the bank. The interior sits at a noticeably higher level than the surrounding land, and within it are the remains of at least two rectangular house-sites. The larger of the two, in the south-west sector, measures roughly 8.2 metres by 4.6 metres internally, with walls about a metre thick and a wide opening of 5 metres in the south-east corner. Stone and earthen ridges extend eastward from it, suggesting additional small enclosures or yards. The second structure, also in the south-west, is more compact at 5 metres by 4 metres internally, but has noticeably thicker walls at around 1.8 metres, with its entrance on the north wall. From its north-east corner a narrow ridge runs northward to meet the low mound that sits just east of that enigmatic depression. The detail comes from C. Toal's North Kerry Archaeological Survey, published in 1995, which catalogued the site but could not resolve what exactly the tunnel feature represents or when any of these structures were built.