This is a fulacht fia, a type of site found in considerable numbers across Ireland, most commonly interpreted as a Bronze Age cooking place. The typical arrangement involved a trough dug into the ground, filled with water, and heated by dropping fire-cracked stones into it. Those stones, shattered and blackened through repeated heating and rapid cooling, were raked out and discarded nearby, building up over time into the characteristic mound that remains. At Knockasarnet, that mound measures roughly seventeen metres east to west and fifteen metres north to south, rising to about 1.1 metres above the surrounding bog surface. Its opening faces west, and some of the burnt material is exposed along the northern portion.

What is particularly worth noting here is the proximity of a rath, a circular earthwork enclosure typically associated with early medieval farming settlements, which lies approximately seventy metres to the east. The fulacht fia and the rath represent quite different periods of activity, the cooking site almost certainly predating the enclosure by well over a thousand years, and yet they share the same patch of landscape. Whether this reflects something meaningful about the site's long-term usefulness to inhabitants, or is simply a matter of usable ground being limited in a marshy area, is difficult to say. The bog itself has done much of the preservation work, keeping the mound intact even as it obscures it beneath vegetation.