They are not unremarkable. Together they represent a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish archaeological record. The term refers to a cooking site, typically Bronze Age in date, where stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil. The cracked and shattered stones were discarded in a heap after use, and it is that accumulation of fire-damaged material that survives as the distinctive horseshoe or oval mound we see today.

At Derrynacaheragh, the main mound on the southern bank is oval in plan, measuring roughly eight metres north to south and just under five metres east to west, and rising to about 1.2 metres in height. A smaller, triangular mound on the northern bank, around 1.5 metres by 1.3 metres, suggests activity on both sides of the stream. The Feabunaun stream itself, which would originally have been the water source that made the site functional, is now the thing most actively working against it. Erosion from the stream is cutting into both mounds and exposing the burnt material within, a slow unravelling that is common on sites where prehistoric deposits sit close to moving water. The juxtaposition is a quietly ironic one: the water that once made the site useful is now gradually dismantling the evidence of it.