What appears at first glance to be a slight swelling in a south-facing hillside is in fact a ringfort, known in Irish as a rath, one of thousands of such enclosures scattered across the Irish countryside. Ringforts were typically used as enclosed farmsteads during the early medieval period, roughly the fifth to twelfth centuries, and their earthen banks and ditches defined a domestic space for a family and their livestock rather than a military stronghold.

The Barnahely example measures roughly 45 metres east to west and 38 metres north to south, making it a fairly substantial but not exceptional example of the type. Its enclosure is formed by a combination of features: an earthen bank, standing about a metre high, runs from the south-west around to the south-east, while a scarp, a natural or cut slope rather than a built-up bank, takes over for the remainder of the circuit. On the outer side, a fosse, essentially a ditch dug to provide material for the bank and to reinforce the boundary, runs from the south-east around to the north-west and reaches a depth of around two metres. The interior ground slopes gently downward toward the south, a detail that may reflect deliberate siting to make use of the natural drainage of the hillside.