It is, in fact, the remains of a fulacht fia, a type of prehistoric cooking or processing site found in great numbers across Ireland, particularly in low-lying ground close to water. The mound itself is the accumulated debris of repeated burning: shattered, fire-cracked stone, discarded after being heated and plunged into water-filled troughs to bring them to the boil. Over centuries of use, this waste material piles up into the characteristic horseshoe or kidney-shaped mound that archaeologists still find scattered across the Irish landscape today.

The Grenagh example measures roughly 8.8 metres north to south and 12 metres east to west, rising to about half a metre in height. Its position on the northern bank of a stream is entirely typical of the type; proximity to a reliable water source was essential to however the site was being used. Fulachtaí fia date broadly to the Bronze Age, though some sites show evidence of use across much of prehistory, and their precise function has been debated for decades. Boiling meat is the long-standing interpretation, but proposals have ranged from textile processing to bathing. Whatever its original purpose, the Grenagh mound has survived into the present as a slight, grassy irregularity in otherwise unremarkable farmland, its interior of blackened, fractured stone hidden just beneath the surface.