The mound is composed not of earth but of burnt and fire-cracked stone, the accumulated debris of repeated high-temperature activity carried out here, possibly over centuries. Two mounds survive at this site: a large oval one measuring roughly 40 metres along its longer axis and standing over two metres high, with a smaller circular companion, less than a metre high, adjoining it on the southern side.

Fulachtaí fia are found in their thousands across Ireland, typically Bronze Age in date, typically sited near water or on boggy ground. The standard interpretation is that stones were heated in a fire and then dropped into a water-filled trough to bring it rapidly to the boil, a method that works efficiently and leaves behind exactly the kind of shattered, heat-stressed stone that forms these mounds. What the boiling water was actually used for remains genuinely contested: cooking, textile processing, leather working, and bathing have all been proposed, and the honest answer is probably that different sites served different purposes at different times. At Carhoo, the boggy ground to the south-east and the presence of a well roughly 30 metres to the north-east fit the pattern well. Water was close, and the repeated use of the site over time built up a mound of considerable size. The smaller adjoining mound may represent a separate phase of activity or a distinct episode of use, though without excavation it is impossible to say with certainty.