It is fourteen metres long, eleven metres wide, and just over half a metre high, with an opening of eight metres facing north-west. What fills that mound, however, is not ordinary earth: it is burnt and fire-cracked stone, the accumulated debris of repeated heating over what may have been centuries.

This is a fulacht fia, a class of monument found in enormous numbers across Ireland, particularly in low-lying, waterlogged ground near streams or springs. The basic principle is straightforward: stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil, after which the cracked and spent stones were discarded to the side. Over time, those discarded stones built up into the characteristic horseshoe shape, with the open end corresponding to where the trough once sat. What fulachta fiadh were actually used for is a long-running debate among archaeologists; cooking meat is the traditional explanation, but brewing, hide preparation, and communal bathing have all been proposed. Most date to the Bronze Age, roughly 1500 to 500 BC, though the form persisted longer in some areas. The Curraghanaltig example sits on the western side of its spring, the water source that would have been essential to the whole operation, and its opening faces north-west across the boggy ground.