It is, in fact, the remains of a fulacht fia, one of the most common prehistoric monument types in Ireland and also one of the least understood. The term refers to a horseshoe-shaped mound of fire-cracked stone and charcoal-rich soil, the accumulated debris of repeated heating. The leading theory is that these sites functioned as cooking places, where stones were heated in a fire, dropped into a water-filled trough to bring it to the boil, and then discarded in a growing pile around the edges. The proximity to water was no accident; streams and marshy ground were essential to the process.

The Coolagh example measures approximately ten metres by nine metres, a moderately sized spread of burnt material that has been grassed over through centuries of disuse. A drain cuts through the spread towards the north-north-west, which may reflect later agricultural management of the wet ground rather than anything prehistoric. Fulachtaí fia are most commonly dated to the Bronze Age, broadly speaking the period from around 2000 BC to 500 BC, though some sites have produced dates outside that range. They tend to cluster near water sources, and their sheer number across the Irish countryside, tens of thousands have been recorded nationally, suggests they were a routine feature of life rather than a ceremonial or exceptional one. What the people of Coolagh were cooking, or whether the site served other purposes such as textile processing or bathing, remains an open question.