A fulacht fia, broadly speaking, is a burnt mound, the accumulated debris of repeated episodes of fire-cracking stone. The typical method involved heating rocks in a fire, dropping them into a water-filled trough to bring it rapidly to the boil, and repeating the process until the cracked and spent stones built up into a horseshoe-shaped mound around the trough. These features are found predominantly in low-lying, often waterlogged ground, and they date mainly to the Bronze Age, roughly 1500 to 500 BC, though some examples extend earlier or later.

What they were actually used for remains a matter of genuine debate among archaeologists. Cooking is the most commonly cited explanation, and experiments have shown that the method works efficiently for boiling large quantities of meat. But proposals have also included brewing, textile processing, and bathing. The ambiguity is part of what makes these sites compelling. They represent repeated, organised activity carried out by communities over generations, yet they leave almost no trace beyond the mound itself and, where conditions allow, the wooden or stone-lined trough beneath it. The Annaghs site in County Kilkenny sits within a county that has a notable concentration of prehistoric activity, from megalithic monuments to ring-ditches and enclosures, and the presence of a fulacht fia there fits a broader pattern of Bronze Age settlement and land use across the river valleys and low ground of the region.