Roughly twelve metres across in each direction, it is made not of earth but of burnt, shattered stone, the accumulated debris of ancient cooking. This is a fulacht fia, a type of prehistoric cooking site found in great numbers across Ireland, typically Bronze Age in date, and usually situated close to a water source. The method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil. The stones cracked and split with the thermal shock, and once spent they were discarded to the side, building up over time into the characteristic horseshoe-shaped or oval mound that survives today.

The Knockglass example follows the pattern closely. Its position on the east side of a stream or drain would have made water easy to draw, and the spread of burnt material at around twelve metres north to south and twelve metres east to west suggests repeated, sustained use rather than a single episode. Ireland has thousands of these sites, making them among the most common archaeological monuments in the country, yet each one represents a specific place where people returned again and again, close to running water, to cook or process something. The reclamation of the surrounding land for pasture over the centuries means the mound has survived largely by accident, its scorched interior protected beneath a skin of grass.