This is a cashel, a type of ringfort defined by a drystone wall rather than an earthen bank, and the example here is substantial in its footprint, measuring roughly 51 metres east to west and 35 metres north to south. It is not, however, intact. Field walls cut across the monument at its north-west and south-east, and gaps on the west and south-south-east sides appear to be modern intrusions rather than original entrances. The result is a site that requires a certain patience to read.

What makes Knockdoebeg particularly interesting is not just the cashel itself but the cluster of structures associated with it. Immediately to the west of the enclosure wall, a poorly preserved rectangular building, eight metres long and five metres wide, sits with its long axis running east to west. The grassed-over stone walls that define it suggest a construction technique broadly similar to the cashel, though whether the two are strictly contemporary is not recorded. A further two structures of comparable size and build lie approximately 25 metres to the west, hinting at a small settlement rather than a single defended enclosure. Cashels of this kind are generally associated with the early medieval period in Ireland, roughly the fifth to the twelfth centuries, when enclosed farmsteads were the dominant form of rural settlement across the country. Adding another layer of context, a second ringfort lies around 230 metres to the north-north-east, suggesting that this part of Knockdoebeg was once a reasonably busy corner of the early medieval landscape.