They appear as low, horseshoe-shaped mounds, typically found beside streams or marshy ground, and for a long time their purpose was genuinely mysterious. The prevailing interpretation today is that they were cooking sites, used during the Bronze Age by heating stones in a fire and dropping them into a water-filled trough until the water boiled. The burnt and shattered stones were then raked aside, gradually building up the distinctive crescent mound that survives in the landscape. The example at Ballygarriff in County Mayo is one such site, a quiet remnant of an activity that was once thoroughly ordinary.

The term fulacht fia translates loosely from Irish as something like "cooking pit of the deer" or "cooking place of the wild animal," though the precise meaning has been debated. What is consistent across excavated examples throughout Ireland is the association with water, fire, and fractured stone. Bronze Age communities, roughly between 2000 and 500 BC, left these features in enormous numbers, suggesting they served a regular and widespread function in daily or seasonal life. Some researchers have proposed alternative uses, including brewing or textile preparation involving hot water, though cooking remains the most widely accepted explanation. The Ballygarriff site sits within a county that has no shortage of such monuments, Mayo's boggy and well-watered terrain being precisely the kind of environment where fulachta fia cluster.