name reflects this folk theory: the word "Fulacht" is obscure but has something to do with cooking, and "Fiadh" means "wild."
the latest science agrees with the folklore:
“Most fulacht fiadh sites are somewhat isolated, with only a few having been found as part of a permanent settlement,” says Emer Dennehy, an Irish archaeologist with the Railway Procurement Agency in Dublin who studies the fulacht fiadh. “We’re still unsure if this indicates a seasonal use with hunting expeditions or if they were used on a regular basis in conjunction with permanent settlements located elsewhere. It was convenient to locate these sites close to wet and damp areas, which would not have been suitable for habitation.”So hunting expeditions are still one of the common explanations for these things. Because they are placed in wet areas, the trough often fills with water by itself; a few are even placed over springs.
Surrounding the troughs are U-shaped mounds made of stones. These mounds can reach heights of more than six and a half feet, though on average they are roughly three feet high, and made of sandstone or limestone. Neither rock type is typically found close to fulacht fiadh sites, indicating that the Bronze Age Irish chose the stones deliberately.Of course people boil lots of different things, not just food but also dying cloth, dirty laundry, and beer -- the main rival to the boiling meat theory is that they were used for brewing.
According to Dennehy, the mounds likely cover hearths where the stones, which show evidence of heat-cracking, were fired. The cracking also provides strong evidence that after being heated, the rocks would be placed in the troughs to heat water. “The stones that were heated and shattered during this process were discarded nearby,” Dennehy explains, “gradually accumulating to form the mound surrounding the trough.”
The water in the trough could have been brought to a boil by adding fired stones, says Dennehy. Demonstrations on modern-day re-creations of fulachtaí fia have shown that intermittently adding one heated rock can keep a steady boil. “You can be absolutely sure it was used for boiling water,” says John Waddell, an emeritus professor of archaeology at the National University of Ireland, Galway. “It’s the one certain thing about the fulacht fiadh.”
The fulacht fiadh are a reminder to archaeologists that people live across the entire landscape, not just in those places where they left enough trash for us to call them "sites." Before the high middle ages, much of Ireland was wild, but it was still heavily used by humans for hunting, gathering, fishing, and communing with the spirits of the land.