It is, in fact, the remains of a fulacht fia, one of the most common prehistoric monument types in Ireland and yet one of the least understood. These horseshoe or oval-shaped mounds, composed of burnt and cracked stone, are found in their thousands across the Irish countryside, typically close to water. The working theory is that they were cooking sites, where stones were heated in a fire and then dropped into a water-filled trough to bring it to a boil. Some researchers have proposed brewing or textile processing as alternative uses, though no single explanation has gained universal acceptance.

The mound at Garranachole measures roughly 24 metres across its longest axis and sits at the base of a south-west-facing slope, on the western side of a pond. A scarp, a low earthen edge left by the accumulated debris of burnt stone, reaches a maximum height of about 0.75 metres along its western and north-eastern sides, with a slighter rise elsewhere. A stream running from the pond passes just to the north of the site. The proximity to standing water is entirely typical; fulachta fia are almost invariably found near streams, springs, or boggy ground, since a ready water supply was essential to however they were used. The burnt, shattered stone that makes up the mound is the material result of repeated heating and rapid cooling, a process that renders the stone useless for further heating and so it was simply discarded to one side, building up over time into the distinctive mound shape still visible today.