This is a rath, the Irish term for a ringfort, the type of enclosed settlement that once numbered in the tens of thousands across the island and served as the basic unit of rural life during the early medieval period, roughly from the fifth to the twelfth century. Most people associate ringforts with bold earthen ramparts and deep ditches, but the one at Colgagh is a quieter example, and its restraint is part of what makes it worth attention.

The enclosure measures twenty-three metres in diameter, defined by a broad bank of earth and stone that varies between roughly three and four and a half metres in width, though it rises only a modest twenty to forty-five centimetres above the interior. There is no fosse, the term for the outer ditch that typically accompanies such banks, visible at ground level. Along the north-east to south-east arc, the bank disappears altogether and the boundary is instead marked by a natural scarp, a low abrupt slope in the ground. This is not damage or neglect but engineering: on the south-east-facing slope where the fort sits, the builders used the existing topography to do some of the work for them, letting the natural fall of the land substitute for a constructed bank where the ground already provided a clear edge. Along the opposite arc, from north to south, the bank grows progressively taller to compensate for the underlying slope and maintain a consistent enclosed space. A gap of one and a half metres in the bank at the north-north-east may be all that remains of the original entrance.