It measures eighteen metres east to west, twelve metres north to south, and rises to just under a metre in height. To anyone walking past, it might read as nothing more than an uneven rise in wet ground. It is, in fact, a fulacht fia, one of the most common yet least understood monument types in the Irish landscape.

Fulachta fiadh are prehistoric cooking sites, typically Bronze Age in date, formed from the accumulated debris of repeated use. The usual method involved heating stones in a fire, dropping them into a water-filled trough to bring the water to a boil, and using that boiling water to cook meat. The discarded, fire-cracked stones built up over time into a mound, often taking the characteristic horseshoe or kidney shape seen here, with the open end facing what would have been the trough or a nearby water source. The marshy ground at Fanahy would have made the site well suited to this purpose, since water would have been close to the surface and easily retained. Roughly twenty metres to the south-east lies a separate burnt mound, a related but distinct deposit of the same fire-shattered stone, suggesting this small area of boggy ground saw sustained activity across some period of prehistoric use.