That detail, small and almost domestic, is what lingers about this site. A fulacht fia is a type of prehistoric cooking or heating site found in great numbers across Ireland, typically consisting of a mound of fire-cracked stone accumulated beside a water trough. Stones would be heated in a fire and dropped into the trough to bring water to the boil, a method that leaves behind a characteristic horseshoe-shaped dump of shattered, heat-reddened rock. At Sonnagh, the mound sat within an east-west peat basin, flanked by higher and drier ground to the north and south, with what may once have been a stream running close by to the south.

The site, designated Sonnagh 3, came to light not through deliberate survey but through the groundwork preceding the construction of the N5 Charlestown bypass. Archaeological testing identified it, and a full excavation followed under ministerial direction. What the excavators found was a roughly oval mound, about ten metres by six metres and just 0.35 metres high, composed of compacted burnt stone set in a charcoal-rich, peaty matrix with lenses of undisturbed peat running through it. Beneath the south-western edge of the mound lay the trough itself, a sub-circular pit cut into the underlying peat, around 1.25 metres in diameter and 0.4 metres deep, with concave sides and a flat base. The alder plank in its base, pinned by two rods, was presumably there to stabilise the floor of the trough or to help with water management. Radiocarbon dating of alder charcoal from the trough fill returned a date range of 2867 to 2497 Cal. BC, placing the site firmly in the late Neolithic period, a time well before the widespread use of metal in Ireland.