These low, horseshoe or kidney-shaped mounds are the residue of prehistoric cooking sites, where stones were heated in fire and dropped into water-filled troughs to bring them to a boil, the cracked and spent stones accumulating over repeated use into the charcoal-dark humps that survive today. The example at Derreenataggart sits in a hollow in blanket bog, in rough hill grazing, its kidney shape measuring just over ten metres north to south and a little over five metres east to west, rising only about forty centimetres above the surrounding ground. That modest height belies what it represents: cycles of fire, heated stone, and water, repeated often enough to leave a lasting mark on the landscape.

The mound's western-facing opening, about one and a half metres wide, points directly towards a marshy area and a small stream, which is exactly where you would expect it to. Fulachtaí fia are almost always found near reliable water sources, and the boggy hollow here would have provided that readily. What makes the Derreenataggart site particularly interesting is that it does not stand alone. A second fulacht fia lies roughly twenty metres to the south, and traces of a field boundary survive about twenty metres to the northwest. That combination, two cooking sites in close proximity with evidence of some form of land organisation nearby, suggests a degree of activity in this upland area that the bleak, open terrain today gives little hint of. The charcoal-enriched soil within the mound points to sustained and repeated burning, the slow accumulation of a place that was returned to, used, and used again.