They are fulachta fia, a term referring to ancient burnt mound sites found throughout Ireland, typically interpreted as outdoor cooking places where stones were heated in fire and then dropped into water-filled troughs to bring them to the boil. The two mounds at Derryleagh are unremarkable to look at now, but their original form was a distinctive horseshoe shape, a profile that is almost a calling card of the type.

Until around 1980, both mounds retained that characteristic horseshoe outline, recognisable to anyone familiar with these sites. At some point during land improvement work, they were levelled, and what had been three-dimensional earthworks became a flattened, grass-covered spread measuring roughly nineteen metres east to west and just over twenty metres north to south. Local memory preserved the knowledge of what had been lost. The site sits on the south-eastern side of a well, with a second fulacht fia immediately to the north, and both are thought to correspond to three sites recorded by a researcher named Bowman in 1934, noted in that year as lying on the land of one E. Guerin. The clustering of multiple fulachta fia in close proximity is not unusual; the sites tend to congregate near water sources, and a well would have made this a practical location across successive generations of use.