Most visitors, if they noticed it at all, would take it for a slight irregularity in the ground. It is, in fact, the remnant of a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland, and one so thoroughly absorbed into its surroundings that some of its burnt material has been built into the adjacent field fence itself.

Fulachtaí fia, the plural form, are among the most commonly recorded prehistoric monuments in Ireland. They typically consist of a horseshoe-shaped mound of fire-cracked stone and charcoal, built up over many uses beside a trough, usually timber-lined, into which water was poured and heated by dropping in stones from a fire. The method was efficient enough to bring water to a boil, and while cooking is the most widely accepted explanation for their use, experiments and ongoing debate have suggested other possibilities, including textile processing or bathing. The Knockearagh example is a modest one: the mound barely registers in the landscape, and the incorporation of its burnt stone into a later field boundary is a reminder of how often prehistoric material has been quietly recycled by subsequent generations of farmers who had no particular interest in what they were handling.