This is a fulacht fia, a type of site found in extraordinary numbers across Ireland, typically identified as a horseshoe-shaped mound of fire-cracked stone built up around a trough. The usual interpretation is that stones were heated in a fire and dropped into a water-filled pit until it boiled, the cracked and spent stones then raked aside to form the characteristic mound. At Knockawillin, what remains is a modest oval of that blackened debris, roughly twelve metres by ten, and only about twenty centimetres high.

The flatness of the mound is not simply the result of time. Local information recorded before 2000 indicates the site was levelled around 1974, most likely as part of the land reclamation work that brought the surrounding pasture into agricultural use. That kind of clearance was common across Ireland during the mid-twentieth century, when drainage schemes and farm improvement grants encouraged the flattening of inconvenient earthworks. The mound's survival at all, even in this reduced form, suggests enough material remained after levelling to leave a readable trace in the soil and vegetation.