It measures roughly six metres long, four metres wide, and barely twenty centimetres high, which is to say it would be easy to walk past without a second thought. But the blackened, fire-cracked material that makes up its bulk marks it as a fulacht fia, one of the most common yet least understood monument types in the Irish landscape.

A fulacht fia is, in its simplest form, a prehistoric cooking place. The standard interpretation is that stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil, and the shattered, heat-spent stones were then discarded in a pile nearby. Repeated over many seasons or generations, that discard heap became the characteristic horseshoe-shaped or irregular mound we see today, typically dark in colour from charring and rich in fragmented stone. These sites cluster around watercourses and marshy ground, which is precisely why the stream at Mahallagh matters here. The form at this site, an irregular rather than neatly horseshoe-shaped mound, is not unusual; many fulachtaí fia lose their original shape over centuries of pasture use, animal movement, and settling ground. They date broadly to the Bronze Age, roughly 1500 to 500 BC, though some examples have earlier or later activity.