Among the more intriguing archaeological sites found in Ireland are those known as fulacht fiadh. They are the most common archaeological sites in Ireland, with over 4,500 recorded examples from every county with the greatest concentration in the South of the country.
Radiocarbon dating indicates that the majority of fulachtaí fia were constructed during the mid to late Bronze Age (c.1500- c. 500 BC), though some Neolithic examples are known. Some of these sites were still in use up to medieval times.
They are generally found close to water sources, such as springs, rivers and streams or waterlogged ground and in the vicinity of sources of readily available stone and fuel. In 1998, Christina Fredengren carried out a survey around a selection of streams and wetlands on both the eastern and western sides of Lough Gara on the border between Sligo and Roscommon. She found that most of these sites were situated on the high plains on the western side of the lake, high up in the landscape.
Around Lough Gara the burnt mounds were mainly found beside small streams and often at heights on the slopes rather than nearer to the lakeshore. They are in the middle of the landscape with respect to height. What all the burnt mounds have in common is their location by what may have been slow-running water.
Christina Fredengren notes that the sites around Monasteraden, on the western shore of Lough Gara, tend to be located near monumental burials such as the ring-cairns and the standing stones, occupying the same position in the landscape. This would suggest that the living to some extent shared this altitude in the landscape with the dead.
Fulachtaí fia generally consist of three main elements: a mound of stones, a hearth used to heat the stones, and a trough, often lined with wood or stone, which was filled with water and into which the heated stones were placed to warm the water. Using this method researchers have found that the water in the trough could be brought to the boil in 30-35 minutes.
It is not known for certain what purpose these sites served and suggested use has included such diverse activities as: outdoor cooking areas, bathing, steam baths, the washing and dying of cloth and even as a means of brewing beer. Some archaeologists have cast doubt on the use of such sites for cooking as no remains of foodstuffs have been found. Other researchers believe the fulachtaí fia were multi-purpose and could have been used for all of these activities.
For example, bathing was seen as an important act in ancient Ireland with a specialised terminology associated with it. Fulachta fiadh may even have acted as centres for ritual or recreational bathing as they were in other countries since prehistoric times.
In August 2007 two Galway based archaeologists suggested that fulachtaí fia were used primarily for the brewing of beer. They experimented by filling a large wooden trough with water and adding heated stones. Once the water had reached approximately 65 degrees Celsius they added barley. After some 45 minutes they transferred the mixture to separate vessels to ferment adding wild plant flavourings and yeast. Three days later, they discovered that it had transformed into a drinkable light ale.
It is not known whether early sites were built by permanent settlements or nomadic hunters. Little evidence of permanent structures has been found in the vicinity of such sites although small hut sites are common. For now at least, these ancient structures remain something of a mystery with their function lost in the mists of time.