That visual distinction, subtle as it sounds, is often the only thing that marks a fulacht fia for what it is: the buried remnant of a prehistoric cooking site, probably Bronze Age in origin, where stones were heated in fire and dropped into a water-filled trough to bring it to the boil. The heat-shattered stone that gives these mounds their characteristic crescent or horseshoe shape was simply thrown aside after each use, building up over time into the low, dark spreads archaeologists find across Ireland to this day.

The mound at Ranagissaun is roughly circular, measuring approximately 12.5 metres north to south and 15.5 metres east to west, with a maximum height of just 0.7 metres. Beneath the grass it is composed of fire-cracked stone packed into a charcoal-rich soil, the two materials together telling a familiar story of repeated burning and discarding across what may have been generations of use. It sits on level ground along the south-western side of a stream or drain, which is typical of the type; a reliable water source was essential to the whole process, and fulachtaí fia are almost always found close to one. The gently rising ground to the south-west would have offered some shelter, though the surrounding field, wet and rush-grown, gives a reasonable impression of what this corner of Mayo has probably always been like.