They appear as low, horseshoe-shaped mounds, typically beside a stream or marshy ground, and they date mostly to the Bronze Age, roughly 1500 to 500 BC. The name, sometimes translated loosely as "cooking place of the deer," points to the most widely accepted theory about their function: that they were used for boiling water by heating stones in a fire and dropping them into a water-filled trough. The stones crack with repeated heating and cooling, and the shattered fragments accumulate into the distinctive burnt mound that survives today. One such monument sits at Ballynagard in County Clare, a quiet addition to a category of site that, despite being one of the most common prehistoric monument types in Ireland, rarely draws much attention.

The mechanics of a fulacht fia are straightforward enough to reconstruct from the archaeology. A wooden or stone-lined trough was sunk into the ground near a water source, filled with water, and then heated using fire-cracked stones. Experiments have shown the method can bring water to a boil relatively quickly and sustain it long enough to cook large joints of meat. Some researchers have argued the sites also served as communal bathing facilities or were used in hide-working and textile production, and the debate has never been fully settled. What is consistent across examples is the proximity to water and the accumulating mound of heat-shattered stone, sometimes called "popped" or "potboiler" stone, that gives these sites their characteristic profile in the field.