They appear as low, horseshoe-shaped mounds, typically dark with charred and fire-cracked stone, sitting in low-lying or waterlogged ground. One such site lies at Killeen in County Mayo, a quiet remnant of prehistoric activity in a county that holds a remarkable concentration of these features. The mounds are the waste heaps of ancient cooking or industrial sites, built up over repeated use as heated stones were plunged into water-filled troughs to bring them to the boil. The stones, fractured by thermal shock, were raked out and discarded, gradually forming the characteristic mound shape that survives today.

The term fulacht fia, loosely translated from Irish as something like "cooking place of the wild deer", reflects older theories about their purpose, though archaeologists have proposed a wide range of uses over the years, from communal cooking and food processing to brewing, hide-working, and bathing. Most date to the Bronze Age, roughly 1500 to 500 BC, though some sites show evidence of earlier or later activity. Their preference for wet, marginal ground meant they were often left undisturbed by later agriculture, which helps explain why so many survive. The Killeen example in Mayo joins a broader pattern of prehistoric land use across the region, where fulachtaí fia cluster in areas that would once have been seasonally occupied grazing land or hunting ground.