They appear as low, horseshoe-shaped mounds, typically dark and waterlogged, and they cluster near streams and marshy ground with a consistency that tells you the water was the whole point. The one at Knockbaun, in County Mayo, is one such site, quietly occupying its place in the landscape without fanfare or interpretive signage.

The term fulacht fia is an old Irish phrase sometimes translated loosely as "cooking place of the deer" or "cooking place of the Fianna," though the name may say more about later folklore than actual function. The monuments date predominantly to the Bronze Age, roughly 1500 to 500 BC, and the working theory for most of the twentieth century held that they were outdoor cooking sites. The method would have involved heating stones in a fire, dropping them into a water-filled trough, and bringing the water to a boil. The mound itself is the accumulated debris of those cracked and discarded stones, built up over repeated use across perhaps generations. More recent scholarship has raised other possibilities, including bathing, textile processing, and brewing, and the honest answer is that no single explanation fits every site neatly. Mayo has a notable concentration of these monuments, which is partly a reflection of the county's extensive bogland, where organic material survives and low-lying, wet ground made such sites both practical and archaeologically preservable.