This is a fulacht fia, a type of site found in enormous numbers across Ireland, and one of the most recognisable signatures of prehistoric activity in the landscape. The term refers to a mound, typically crescent or U-shaped, formed from the accumulated debris of repeated episodes of fire-heating stones and dropping them into a water-filled trough to boil or cook. The burnt, fractured stone was simply raked aside each time, building up over generations into the distinctive curved heap that survives today.

The Knockglass example was inspected in 2012 and found to measure up to 13.7 metres from east-northeast to west-southwest and 9.4 metres from north-northwest to south-southeast, reaching a maximum height of 1.55 metres. The opening faces north-northwest, as is common with many such sites, and the gap at the centre of the mound is approximately 4.5 metres wide. The mound material is the familiar combination of burnt stone and charcoal-rich soil that gives fulachtaí fia their dark, slightly acrid character even after millennia. Most examples in Ireland date to the Bronze Age, roughly 1500 to 500 BC, though some sites span earlier or later periods. Their precise function has been debated by archaeologists; cooking is the most widely accepted explanation, but brewing, bathing, and textile processing have all been proposed, and these uses need not have been mutually exclusive.