It is about ten metres long, seven metres wide, and less than a metre high, which makes it easy to walk past without a second glance. On its northern side, a smaller raised area of roughly square outline adds a further subtle irregularity to the ground. What lies beneath is the accumulated debris of repeated prehistoric cooking, a fulacht fia.

A fulacht fia, sometimes called a burnt mound, is among the most common prehistoric monuments in Ireland, yet most people have never heard of one. The basic method involved heating stones in a fire, then dropping them into a water-filled trough, usually timber-lined, dug into the ground. The heat transferred to the water was sufficient to cook meat or, some researchers now argue, to brew ale or process hides. The stones, once cracked by the thermal shock, were discarded to one side, and over centuries of repeated use those discarded fragments built up into the horseshoe-shaped or kidney-shaped mounds that survive today. The marshy setting at Knockacareigh is entirely typical: fulachta fia are almost always found close to water or in low-lying, wet ground, where a trough could be filled easily. Most date to the Bronze Age, broadly between 2000 and 500 BC, though some examples are earlier or later.