It is, in fact, a burial cairn, roughly fifteen metres across and just over a metre high, defined by a kerb of thin flat flags with a row of vertical slabs set inside them. The whole structure sits on an exposed shelf of limestone bedrock, probably because the surrounding rock has eroded away over the millennia, leaving the cairn slightly elevated on its platform in a way that was not originally intended.

The cairn's interior tells a more precise story. Farm improvement works in the mid-1990s damaged the eastern half of the monument, and a partial excavation carried out in 1997 under the direction of archaeologist James Eogan revealed a cist at the centre, a small stone-lined box burial of the kind common in prehistoric funerary practice, containing both inhumed and cremated bone. A separate inhumed burial was found within the body of the cairn itself. Radiocarbon dating placed the central cist in the Chalcolithic period, roughly 2460 to 2140 cal BC, a transitional era between the Neolithic and the Bronze Age when copper was beginning to be worked in Ireland. The later burial dated to the Early Bronze Age, between 1880 and 1610 cal BC. Among the objects recovered were a perforated dog tooth, a glass bead, a chert scraper, a flint blade, and a sherd of pottery subsequently assigned to the Beaker tradition, a distinctive ceramic style associated with communities spread across much of Atlantic Europe during this period. Perhaps as notable as the burials themselves is what the excavation revealed about the landscape: the cairn appears to be contemporary with the field system that surrounds it, suggesting that the people who farmed this ground and the people who buried their dead here were the same community, living and working within a single organised landscape. That landscape has since been compared with Early Bronze Age remains on Roughan Hill and the Carran Plateau, placing Coolnatullagh within a broader pattern of prehistoric settlement across the Clare uplands.