They read in the landscape as low, kidney-shaped or oval mounds, typically dark with charcoal-enriched soil and packed with heat-shattered stone. The one at Derreenataggart sits in a hollow in rough hill grazing, a modest oval measuring roughly 7.5 metres north to south and 6 metres east to west, rising only about 45 centimetres above the surrounding ground. Along its western side there is a slight indentation, the kind of subtle topographic detail that rewards a second look.

A fulacht fia, broadly understood, is the debris mound left behind by a Bronze Age hot-water trough. The working interpretation is that stones were heated in a fire, then dropped into a water-filled pit to bring the liquid to a boil, and the cracked, spent stones were discarded to the side, building up the mound over repeated use. What exactly the hot water was for, cooking, bathing, textile processing, or some combination, remains a matter of genuine debate among archaeologists. At Derreenataggart the site does not stand alone. A second fulacht fia lies approximately 20 metres to the north, and a field boundary survives around 40 metres to the northwest. The proximity of these features to one another suggests this patch of boggy hillside saw sustained and perhaps varied activity during prehistory, even if the precise relationship between the monuments is now impossible to recover.