What it actually preserves is the footprint of a Bronze Age cooking technology that was once one of the most widespread features of the Irish landscape. A fulacht fia, in its simplest form, is a burnt mound, the accumulated debris of a process in which stones were heated in a fire and then dropped into a water-filled trough to bring it to a boil. The site at Ballynaguilla measures roughly 18.7 metres east to west and 13.8 metres north to south, an oval spread of blackened, fire-cracked stone that is still visible in cross-section where the stream has cut into its bank. A small mound, about three metres across and 0.8 metres high, sits at the eastern end of the spread.

What makes Ballynaguilla more than a field anomaly is the trough itself, investigated by Bowman in 1934. The wooden structure he uncovered was precise and considered: six feet two inches long, three feet one inch wide, and two feet one inch deep. The base was built in two oak planks of different widths, while the sides were formed from hazel branches, each roughly five inches in diameter, five to a side, still carrying their bark when exposed after what may have been thousands of years in waterlogged ground. The ends of the trough were held in place by the projecting sides, and the whole assembly was anchored by posts driven into the earth at each corner. The choice of materials was deliberate: oak for the base, dense and durable; hazel for the sides, flexible and workable. It is a construction that rewards close reading, not a rough pit dug into the ground, but a fitted, jointed vessel built to hold water under sustained heat.