It is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, typically dating to the Bronze Age. The usual explanation is that stones were heated in a fire and then dropped into a water-filled trough to bring it to a boil, cooking meat wrapped in straw or hide. The process generated vast quantities of shattered, heat-reddened stone, which accumulated over repeated use into the distinctive horseshoe-shaped mounds that archaeologists still find in their thousands today.

This particular example is roughly oval in plan, measuring about 20 metres north to south and 23 metres east to west, and standing 1.65 metres at its highest point. At its centre is a depression approximately 5.5 metres across and 1.2 metres deep, which likely marks where the trough once sat. The eastern edge of the mound has been cut through by a stream, which is not unusual; fulachta fia are almost always found close to a reliable water source, and the stream here may well have been the very reason the site was used in the first place. What makes Coolbaun slightly more notable is that a second fulacht fia lies only about 20 metres to the north-north-east, suggesting the area saw repeated or sustained activity during prehistory, whether by the same community across generations or by different groups drawn to the same convenient combination of water, fuel, and open ground.