A subrectangular depression, roughly eleven metres long and almost half a metre deep, marks where the roof of an underground passage has given way over centuries. Most visitors to ringforts, the circular earthen enclosures known in Irish as raths that were built across Ireland during the early medieval period, would never suspect a tunnel running beneath their feet. At Liscottle, a discreet lintelled opening at the north end of the collapse lets you see directly into one.

The opening is modest, less than sixty centimetres wide and forty centimetres high, but beyond it the passage is surprisingly coherent. The walls are built of rough, random rubble and are slightly corbelled, meaning the stones are angled progressively inward toward the top to bear the weight of the flat roof slabs above them. The floor is earthen and scattered with loose stones. From the entrance, the passage curves gently for approximately 6.6 metres toward the northwest, heading in the direction of the rath's outer bank, or scarp. At that far end, another lintelled opening is still visible in the scarp face, though it is now blocked. Souterrains, as these underground stone-lined passages are known, are a recurring feature of early medieval raths across Ireland, and were likely used for storage or as places of refuge. What is relatively unusual at Liscottle is the combination of a still-intact navigable section alongside the clearly visible evidence of collapse, so that both the structure and its deterioration can be read in the same glance. A large stone sits at the southern end of the depression, possibly a displaced roofing slab, marking where the passage gave out entirely.