Ordnance Survey cartographers recorded it by that name in both 1842 and 1915, which suggests it was a known and recognisable feature across generations, even as its original purpose quietly faded from understanding.

The structure is a souterrain, a type of dry-stone underground passage or chamber common in early medieval Ireland, typically associated with a ringfort or cashel and thought to have served for storage, refuge, or both. A cashel is a stone-walled enclosure, the equivalent of an earthen ringfort, and this one still stands at Mortyclogh. The souterrain within it consists of a single chamber nearly nine metres long and just under two metres wide, with a clay floor, a rounded western end, and stone lintels forming the roof. Near the western end, a narrow square opening leads off through a tight passage called a creep, only about eighty centimetres wide and traceable for roughly two and a half metres before its southern end fills completely with clay. When a writer named Cooke visited in 1851, the picture was different: he was able to access two chambers connected by one of these creeps, the second running perpendicular to the first and roughly the same size. That second chamber is now effectively sealed. Cooke also noted the discovery of what he called a fairy millstone, a small round stone about an inch and a half across. He did not record whether it had a central hole, but the description fits a spindle whorl, a small weighted disc used to keep a hand spindle spinning during thread-making, and a reasonably common find on early medieval sites.

The current entrance, at the north-eastern end of the chamber, shows a displaced lintel, and it is uncertain whether this was always the way in or whether it was originally another creep connecting to a further space now lost. The clay that fills the southern passage may yet conceal a second chamber, and the question of what Cooke saw and what now lies buried remains, in a modest but genuine sense, open.