It measures just under twelve metres across and less than a metre high, with a shallow depression at its centre, and to an untrained eye it might read as nothing more than a natural rise in boggy terrain. In fact it is a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish archaeological landscape.

Fulachtaí fia, the plural form, are the remains of ancient cooking or processing sites, typically Bronze Age in origin, found in their thousands across Ireland. The standard interpretation is that they functioned as outdoor cooking places: a trough dug into the ground would be filled with water, then heated by dropping fire-cracked stones into it until the water boiled. Those shattered, heat-spent stones were discarded to the sides over time, building up the characteristic horseshoe or circular mound of burnt material that survives today. The central depression at Knockawillin, measuring roughly 2.8 metres east to west and 3.3 metres north to south, with a depth of around 0.4 metres, is consistent with just such a trough. The choice of marshy ground was deliberate rather than incidental; a high water table made it far easier to maintain a filled trough without the need to carry water from a distance.

The site is partially overgrown, which is the condition in which most fulachtaí fia are encountered. The mound itself is the monument; there is rarely anything visually dramatic about these places, and that is part of what makes them quietly compelling. They are the residue of repeated, ordinary activity, possibly spanning generations, preserved simply because burnt stone and boggy ground have a way of enduring together.