The one at Treanrevagh, in County Mayo, is a quiet example of a type that appears so frequently it can seem almost unremarkable, yet each site carries the same fundamental strangeness: a horseshoe-shaped mound of fire-cracked stone, typically beside a stream or marshy ground, left behind by people who returned to the same spot again and again to heat water by dropping superheated stones into a trough.

The term fulacht fia is an Old Irish phrase sometimes translated loosely as "cooking place of the deer," though what these sites were actually used for has been debated for decades. The burnt-stone mound is a by-product of the process: once a stone had been heated in a fire and plunged into water, it cracked and became useless for further heating, and so the spent fragments accumulated over time into the low, distinctive humps that survive today. Most fulachtaí fia date to the Bronze Age, roughly 1500 to 500 BC, though some were used earlier or later. Beyond cooking, scholars have proposed uses ranging from textile processing to bathing to brewing, and it is possible that different sites served different purposes at different times. The Treanrevagh example sits within a Mayo landscape that has yielded many such monuments, the boggy, low-lying terrain of the west having preserved them particularly well.