It is roughly twenty metres long, nearly eighteen metres wide, and less than a metre high, with an opening eight metres across facing north. The material forming it is burnt, cracked stone, which is exactly the point. This is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, and the sheer ordinariness of its appearance is at odds with the millennia of use it may represent.

Fulachtaí fia, the plural form, are among the most common archaeological monuments in the Irish landscape, yet they remain quietly puzzling. The standard interpretation is that they functioned as outdoor cooking places, probably during the Bronze Age, though some examples have been dated earlier or later. The method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil. The stones shatter with repeated heating and cooling, and over time the discarded fragments accumulate into exactly the kind of low, curved mound visible at Barleyhill. The proximity to a stream is characteristic; a reliable water source was essential to the whole operation. Some researchers have proposed alternative uses, including bathing or textile processing, though cooking remains the most widely accepted explanation. The horseshoe or kidney shape of the mound, open on one side, typically reflects the position of the trough at the centre of the activity.