The example at Knockduff in County Cork sits in rough grazing land, presenting itself as an L-shaped mound of dark soil studded with small shattered fragments of quartzite and limestone. It measures nineteen metres along its longer axis and reaches just 0.7 metres in height, the kind of low, unassuming rise that could easily be mistaken for a natural quirk of the ground. Its opening, fourteen metres wide, faces south-east, and a dry channel two and a half metres wide extends away from the site towards the south-south-west, the ghost of whatever water source once made the whole operation possible.

A fulacht fia, in its simplest form, is a burnt mound associated with the heating of water. The prevailing interpretation is that stones were heated in a fire and then dropped into a water-filled trough, bringing the water rapidly to a boil. The shattered, heat-cracked stones were then discarded into a growing heap around the trough, which is what gives these sites their characteristic crescent or horseshoe shape, and in this case, an L-shaped profile. The dark colouration of the soil is a byproduct of prolonged burning over many seasons or generations. These sites date broadly to the Bronze Age, though their precise function remains a matter of debate; cooking, bathing, textile processing, and hide preparation have all been proposed. What makes Knockduff quietly notable is that it does not stand alone. A second fulacht fia lies approximately ninety metres to the south-east, and several limestone boulders are scattered in the surrounding area, suggesting this corner of North Cork saw repeated or sustained prehistoric activity rather than a single isolated event.