The place exists in the record only because a plough disturbed it sometime in the 1970s, turning up the characteristic signs of a fulacht fia: burnt stone, charcoal, and soil darkened by centuries of accumulated use. Since then it has sunk back below ground level, invisible to anyone walking the gentle south-east-facing slope where it lies, close to a cluster of natural springs.

A fulacht fia is a type of prehistoric cooking site found in enormous numbers across Ireland, typically Bronze Age in origin. The standard interpretation is that stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil, allowing meat to be cooked without direct flame. The crescent-shaped mounds of shattered, fire-cracked stone that result from this process are among the most common archaeological features in the Irish landscape, and yet each one represents repeated, organised activity by real communities over what were sometimes considerable periods of time. The Johnstown example sits roughly 150 metres south of a related site, which suggests this particular stretch of ground was a meaningful place in the prehistoric landscape, probably because of the springs nearby. Access to a reliable water source was essential to how a fulacht fia functioned, and this south-east-facing slope, well-watered and sheltered, would have made practical sense to whoever used it.