It is not obviously remarkable to the passing eye, but beneath its grass covering lies dark, fire-stained soil and a compact layer of sandstone, at least one piece of which appears to have been heat-shattered. This is a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland, typically identified by exactly this combination: a crescent or horseshoe-shaped mound of burnt and discarded stone, positioned close to a reliable water source.

Fulachtaí fia (the plural form) were in use primarily during the Bronze Age, roughly 1500 to 500 BC, though some sites span a wider range. The working principle involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil for cooking. The cracked, fire-damaged stones were thrown aside after use, and over time this waste material accumulated into the distinctive mound shape that survives today. The site at Ballynomasna sits within a marshy area recorded on the second edition Ordnance Survey six-inch map of 1907 under the placename 'Cloghaun', on ground described as liable to flood. The mound itself is substantial, measuring roughly 22 metres east to west and 17 metres north to south, with a slight depression near the western end of its southern side that gives it the characteristic kidney outline, though the mound extends further eastward than west. It was first formally identified as a monument through an aerial photograph, catalogued under a Bord Gáis reference, which speaks to how many such features remain invisible at ground level until seen from above, especially in flat, reedy terrain where a low mound can disappear into the landscape entirely.