It measures roughly twenty metres east to west and eleven metres north to south, rising only about twenty-eight centimetres above the surrounding ground. Easy to overlook, easy to mistake for a natural undulation in the field. But this modest hump is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, and its proximity to the well is no accident.

Fulachtaí fia, the plural form, are among the most common archaeological monuments in the Irish landscape, with thousands recorded nationwide. The prevailing interpretation is that they functioned as outdoor cooking places, probably during the Bronze Age. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring it to boiling point. The cracked, heat-shattered stones were discarded in a pile around the trough, and it is precisely this accumulated debris, stained dark by repeated burning, that forms the horseshoe-shaped or oval mounds visible today. The well immediately to the south of this example would have provided a reliable water source, which explains the siting neatly. About 140 metres to the north lies a ring-barrow, a low circular earthwork enclosing a burial, suggesting that this corner of Castlewrixon was a place of some significance across a broad span of prehistory, with the living, the dead, and the practical business of subsistence all leaving their marks within a short distance of one another.