The mound is horseshoe-shaped, roughly sixteen metres long and twelve metres wide, rising only about eighty centimetres above the surrounding ground. Its opening, nearly two metres across, faces south. A few metres to the west, a second mound of the same type sits close enough to suggest the two were used in tandem, or at least by the same community across the same stretch of time.

This is a fulacht fia, a class of monument found in enormous numbers across Ireland, particularly in low-lying and waterlogged ground. The term is generally understood to refer to a prehistoric cooking site, though the precise function has been debated: some archaeologists favour communal meat-boiling, others have proposed brewing, hide-working, or bathing. The method, whatever the purpose, was consistent. Stones were heated in fire and then dropped into a water-filled trough, raising the temperature rapidly. The repeated heating and quenching shattered the stones, and the discarded fragments accumulated over time into the characteristic burnt mounds that survive today. Most date to the Bronze Age, roughly 1500 to 500 BC, though examples from other periods are known. The uneven surface of the Lyroe mound reflects centuries of accumulation and the disturbance left behind by drainage works that partially destroyed it.