To the untrained eye it reads as nothing at all, just a slight discolouration or irregularity in the ground. In fact it marks the site of a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish archaeological record.

A fulacht fia is essentially an ancient cooking place, typically comprising a trough dug into the ground, a hearth for heating stones, and a mound of those same stones once they had been used and discarded. The method works by heating the stones in a fire and dropping them into a water-filled trough until the water boils. The shattered, fire-cracked stones accumulate into a horseshoe-shaped mound over time, and it is that mound, stained dark by burning, that tends to survive. Such sites are found in enormous numbers across Ireland, most commonly dating to the Bronze Age, and they are almost always located close to a water source, as the one at Knocknaneirk is, sitting southwest of a stream that would have supplied the necessary water.