They appear as low, horseshoe-shaped mounds, typically found near water, and are generally dated to the Bronze Age, roughly 1500 to 500 BC. One such site sits at Clenagh in County Clare, quietly occupying its place in the landscape without much in the way of fanfare or signage.

The working theory behind fulachtaí fia, developed and refined over decades of excavation, is that they functioned as outdoor cooking sites. The typical arrangement involved a timber-lined trough sunk into the ground near a stream or marshy area, which would be filled with water and then brought to a boil by heating stones in a nearby fire and dropping them into the trough. The shattered, heat-cracked stones, discarded after use, are what built up over time to form the distinctive crescent mound we see today. Some researchers have proposed alternative uses, including textile processing, leather working, or bathing, and it is possible that different sites served different purposes at different times. The Clenagh example is one of a considerable concentration of such monuments recorded across Clare, a county whose boggy ground and river margins offered exactly the damp, low-lying conditions these sites seem consistently to favour.