The enclosure at Coolboy measures roughly 60 metres north to south and 50 metres east to west, making it a substantial feature on the landscape, yet it went unrecorded until relatively recent times.

The site occupies the southern end of a slight north-south ridge, its western flank facing downslope in a manner typical of enclosures placed to command a view across lower ground. What defines it is a fosse, a rock-cut or earth-dug boundary ditch, running around its perimeter and measuring approximately four metres in width. A fosse of that scale suggests deliberate and labour-intensive construction, though without excavation the enclosure's date and function remain open questions. It was first reported by Jean Charles Caillére, and its outline is now detectable through aerial imagery, sitting only about 20 metres from a lane that appears on historical maps of the area, close enough that generations of local people would have passed by it without knowing quite what they were skirting.