Archaeologists have recorded and excavated a large number of these sites, which usually appear as a mound of stones, often horseshoe shaped, along with a wood or stone lined trough that is dug into the water table.
Folklore from the medieval period describes these sites as cooking sites for travelling hunting parties, and the common understanding is that they were used tor boiling meat by heating stones in the fire, and dropping them into the water. The water was added from a nearby well or stream, or in low-lying land the trough would fill on its own.
In recent years, archaeologists have questioned the widespread acceptance of these features as cooking sites. There are many more efficient ways to cook food, that involve the use of far less resources and labour, and archaeological evidence for cooking and food consumption, such as animal bones, is scant at best. New possibilites, such as the use of Fulachta Fia as bathing sites and saunas, or areas of industrial processing of wool or leather, are now being considered. One team of archaeologists is also exploring the idea that they can be used for the brewing of beer, and experiments have produced quite intoxicating results! More information can be found here and here.
Dating these sites ranges from the Neolithic period through to the medieval period, although the primary period of use appears to be the Middle Bronze Age, (c. 1500 BC). In the Elizabethan period, walking parties would reuse the sites as a novelty.