High above the Mahon Falls in the Comeragh Mountains of County Waterford, a small circular ruin sits on a rocky knoll with no obvious explanation attached to it. It is not a castle, not a church, not a field boundary. It is simply a roughly four-metre-wide ring of dry-laid limestone rubble, its purpose unrecorded and its builder unnamed, occupying a perch of elevated ground in terrain that most people pass through only to reach the waterfall below.

The structure sits in a small river valley at altitude, with the River Mahon running roughly northwest to southeast about 350 metres to the north. The walling is sub-circular and constructed in roughly coursed drystone, a technique in which stones are stacked without mortar, relying on their own weight and careful placement to hold form. The style and materials suggest a date somewhere in the eighteenth or nineteenth century, which places the building well within the post-medieval period but still leaves its function open to speculation. The knoll it occupies may itself have been shaped or built up slightly to provide a more level or prominent foundation. It was identified and reported by Helen Lawless, whose photographs remain the primary record of the structure as it currently stands.