It is a fulacht fia, a type of monument found in considerable numbers across Ireland, typically consisting of a horseshoe-shaped or kidney-shaped mound of fire-cracked stone and charcoal, the accumulated debris of repeated heating and cooling. The mound at Cromoge measures roughly 8.4 metres east to west and 7.7 metres north to south, rising to about 0.9 metres at its highest point. It is partially overgrown now, blending into the surrounding pasture, but its form remains legible.

Fulachtaí fia, as the plural goes, are among the most common prehistoric monuments in Ireland, yet they remain genuinely mysterious. The leading theory is that they functioned as cooking sites: stones were heated in a fire, dropped into a water-filled trough to bring it to the boil, and used to cook meat. But brewing, hide-working, and communal bathing have all been proposed as alternatives or additional uses, and the debate has not been settled. What is consistent across these sites is the kidney or horseshoe shape, formed because the mound builds up around three sides of a central trough, leaving an opening. At Cromoge, that opening faces north. The mound is described as circular at the base but rising to a kidney shape toward the top, which likely reflects how the debris accumulated over successive uses of the site.