To a passing eye they might read as nothing more than slight rises in the ground, but they are the surviving traces of a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland. The term refers to a horseshoe-shaped mound of burnt and shattered stone, the debris that accumulated over repeated use of an outdoor hearth and water trough. The standard method involved heating stones in a fire and then dropping them into a water-filled pit or wooden trough to bring the water to a boil; the stones, cracking under thermal stress, were discarded to the sides, gradually building up the characteristic mound.

The site at Cloonsillagh follows the pattern typical of such monuments. A stream runs along the southern side of the field fence, and the two mounds lie just to the north of it, which is exactly where one would expect to find them. Access to a reliable water source was the primary requirement for this kind of activity, and the consistent association of fulachtaí fia with streams, springs, and boggy ground is one of the most recognisable features of the type. The mounds are catalogued as a pair, the second recorded separately, suggesting the location was used more than once or that activity here was substantial enough to generate two distinct accumulations of material. Most fulachtaí fia date to the Bronze Age, roughly between 2000 and 500 BC, though some have produced dates outside that range.