This is a fulacht fia, a type of monument found across Ireland in their thousands, and one of the more enduring puzzles in Irish archaeology. The term refers to a horseshoe or crescent-shaped mound formed from the accumulated debris of repeated heating: stones were fired in a hearth, then dropped into a water-filled trough to bring it rapidly to the boil. Over time, the heat-shattered fragments were raked out and piled up, creating the distinctive mound we see today. What the process was actually used for, whether cooking, textile preparation, bathing, or something else entirely, remains a matter of lively debate among archaeologists.

The mound at Maulnahorna is roughly circular, measuring 17 metres in length, 18 metres in width, and rising to 1.7 metres at its highest point on the eastern side, with a gradual incline from west to east. At its centre sits a depression measuring approximately 2.2 metres north to south and 3.2 metres east to west, which likely marks the position of the original trough. Several trees have taken root in the centre of the mound over time, their roots now threading through layers of fire-cracked stone. The mound's scale, both in area and height, suggests this was a site used repeatedly and perhaps over a considerable period, though without excavation it is impossible to say more precisely when it was in use. Fulachta fia are generally associated with the Bronze Age, broadly spanning from around 2500 to 500 BC, though some sites have produced earlier or later dates.