It is, in fact, the ghost of an underground passage, a souterrain that has largely fallen in on itself over the centuries, leaving only a long, gentle hollow to mark where it once ran.

Souterrains are stone-lined underground tunnels associated with early medieval ringforts, and were likely used for cold storage, refuge, or both. The ringfort here, known as a rath, a circular earthwork enclosure typical of early Irish settlement, still survives, and this souterrain occupies its north-eastern quadrant. The passage ran roughly north to south and extended to around thirteen metres in length, built in the drystone technique, meaning the walls were laid without mortar, relying entirely on carefully fitted stones for their structure. Today only the southern four metres remain intact, with a width of just over a metre, enough to suggest the confined, deliberate architecture of the original construction. An air-vent is still visible in the southern wall, a small detail that speaks to a thoughtful design intended for actual use rather than mere concealment. The rest of the passage has collapsed, and its course is now readable only as that grass-filled hollow trailing northward across the ground.