A fulacht fia is a prehistoric cooking site, typically consisting of a horseshoe-shaped mound of fire-cracked stone built up over centuries of use, usually positioned beside a water source. The burnt and shattered stone was the byproduct of heating rocks in a fire and dropping them into a water-filled trough to bring it to the boil, a process repeated again and again over generations until the discarded material accumulated into a mound. At Cloonsillagh, what survives above ground is modest: a spread of that characteristic dark, heat-fractured stone, now grassed over and easy to overlook.

The site was once more visible than it is today. According to local information, the mound was levelled around 1964, reducing what would have been a recognisable earthwork to the shallow, diffuse spread that remains. A slight linear depression running north to south immediately to the west of the spread may indicate the former position of the trough, the cut or constructed hollow into which water was poured and heated. The levelling of the mound will have disturbed the stratigraphy, but the footprint of burnt material persists as a faint record of repeated prehistoric activity at this spot.