They appear most often as low, horseshoe-shaped mounds, typically brownish and waterlogged, sitting in damp ground near streams or marshy hollows. The one recorded at Carrowbeg in County Mayo is one such site, a fragment of prehistoric activity preserved in the boggy terrain of Connacht.

Fulachta fia, the term drawn from Old Irish and meaning roughly "cooking places of the deer" or "cooking places of the wandering people", are generally dated to the Bronze Age, though some examples extend into the Iron Age. The working theory, supported by experimental archaeology, is that they functioned as outdoor cooking sites. A trough, often timber-lined or cut into the earth, would be filled with water, and stones heated in a nearby fire would then be dropped in to bring the water to a boil. The shattered, fire-cracked stones that accumulate from this process form the characteristic crescent mound that survives today. Some archaeologists have proposed alternative uses, including bathing, textile processing, or brewing, and the honest answer is that a single explanation may never account for all of them. What is consistent is the setting: low-lying, wet ground, close to a water source, which the Carrowbeg location fits neatly into the broader pattern seen across Mayo and the west of Ireland.