Traditionally fulachtaí fia or burnt mounds have been interpreted as ancient cooking places dating back to the Bronze Age. In reality they are a much more complex monument with examples recorded from as early as the Neolithic and right into the Iron Age. These are the most common prehistoric field monuments in Ireland with thousands coming to light in recent years during archaeological excavations carried out in advance of the national road building programme.
An Irish burnt mound usually consists of a low horse-shoe shaped mound of heat shattered stone generally located in boggy ground or adjacent to a stream or river. Excavation usually reveals that the mound is associated with a number of troughs or water basins which can be simply dug into the sub-soil or lined with timber planks or stone slabs. Sometimes the foundations of structures are also identified.
These monuments had a clear primary function; they served to heat water. The troughs or basins were dug into the ground below the water table which caused them to flood naturally. Stone was gathered in great quantities and heated in fires. The hot stone was added to the water trough(s) causing the water to heat or boil. When the hot stone hit the cooled water it shattered. After use the shattered stone was cleared out of the troughs and stockpiled adjacent to the trough, thus resulting in the typical horse shoe shaped mound.
While the primary function is clear (heating water) the purpose to which the hot water was used is the subject of much debate in Irish archaeology. The traditional view of cooking places has been put to the test and it is certainly possible to cook a joint of meat in a trough. Structures found associated with burnt mounds have also been identified as saunas or sweat-houses. Other plausible interpretations include brewing, dying and tanning but the reality is that burnt mounds or fulachtaí fia provided a means of producing hot or boiled water on an industrial scale.