Set on a steeply sloping mountainside above the Keerglen River in County Mayo, this roughly circular stone enclosure was recorded on the 1922 Ordnance Survey six-inch map under the name Doondragon, or the Dragon's Grave. A cashel is a ringfort built from stone rather than earthwork, and this one measures some 26.2 metres across, its drystone walls now largely grassed over and reduced in places to foundation level, particularly along the northern edge where the river's steep bank presses closest. Within the cashel wall, on the south-south-west side, there is a souterrain, an underground passage or chamber of the kind commonly built in early medieval Ireland, often for storage or refuge. It is this feature that almost certainly gave rise to the name. Local tradition held that inside the cashel lay a Giant's Grave, covered with huge stone flags and connected by a passage running from the river bank into the fort itself. That description fits a souterrain precisely, and somewhere in the retelling, the giant became a dragon.

The site carries some weight beyond its unusual name. Writing in 1961, a scholar named Aldridge pointed out that the cashel sits along an ancient routeway through the Keerglen River valley, and proposed that this may have been the very route followed in the Táin Bó Flidhais, a medieval Irish saga describing a cattle raid in this part of Connacht. The Táin Bó Flidhais is a lesser-known counterpart to the more famous Táin Bó Cúailnge, and concerns the goddess or noblewoman Flidhais and her legendary herd of cattle. Whether or not the identification is correct, the cashel sits in landscape that was clearly in use for a long time; a small cairn lies 25 metres to the south, and a further enclosure sits roughly 100 metres to the east, suggesting this was never quite as isolated a place as the rough terrain might imply.