It is the kind of feature that most people would walk past without a second glance, yet beneath it lies a souterrain, an underground stone-lined tunnel or chamber typically associated with early medieval settlement in Ireland, used variously for storage, refuge, or both.

The underground feature sits in or beside what may be a ringfort, the circular earthwork enclosures that were the primary unit of rural settlement in early medieval Ireland, numbering in the tens of thousands across the country. The souterrain itself was noted in 1896, when a source recorded a cavern extending at least six feet before narrowing into a smaller passage beyond. That brief description is essentially all that is known about its interior. Whether the passage continues further, and what condition it is now in, remains unclear. The site is recorded as inaccessible, and the small surface opening that hints at its location offers no useful entry point.

There is something quietly odd about a place defined almost entirely by what cannot be reached. The 1896 account gives just enough detail to confirm that something is there, a chamber, a constriction, a continuation of unknown length, but not enough to say much more. For now, Coolmona keeps its underground passage more or less to itself.