It measures eleven metres long, nine metres wide, and rises just under a metre from the ground. What looks, at a glance, like a natural rise in the field is in fact a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland, and particularly dense in Munster. The term refers to a mound formed almost entirely from heat-shattered stone, the discarded debris of repeated use over generations.

The mechanics of a fulacht fia are straightforward. Stones were heated in a fire and then dropped into a water-filled trough, usually timber-lined, to bring the water rapidly to a boil. The thermal shock caused the stones to crack and splinter, rendering them useless for reheating, and so they were tossed aside. Over time, that discard accumulated into the characteristic horseshoe or oval mounds we see today. The waterlogged ground at Garrynagearagh is typical of the settings these sites favour; the natural presence of water made them practical locations, and the boggy conditions have helped preserve the burnt material over millennia. Most fulachtaí fia date to the Bronze Age, roughly 1500 to 500 BC, though some have produced earlier or later dates. Their precise social function is still debated, with cooking the most widely accepted explanation, though brewing, bathing, and textile processing have all been proposed by researchers at various points.