It measures nine metres long, seven metres wide, and barely forty centimetres high, which means that in a field of grazing animals it could easily pass for a natural rise in the ground. It is a fulacht fia, a type of monument found in their thousands across Ireland, and one of the most persistently mysterious categories of archaeological site on the island.

The term fulacht fia, sometimes rendered fulacht fiadh, refers to these characteristic spreads or mounds of heat-shattered stone, typically found near water. The leading theory is that they functioned as cooking places: stones were heated in a fire, then dropped into a water-filled trough, bringing it rapidly to the boil. The process leaves behind exactly what survives here, a horseshoe-shaped or oval dump of fragmented, thermally-stressed rock. The proximity to a drain or watercourse is entirely typical; nearly every fulacht fia sits close to a reliable water source, and the one at Garrynagearagh is no exception, lying on the western side of a drain that would have supplied the necessary water. Most of these sites date to the Bronze Age, broadly spanning from around 2000 to 500 BC, though some have produced dates outside that range.