A raised, roughly oval platform sits in open pasture, its enclosing earthen bank still running to nearly two metres in height on the southern side, where the builders piled the material highest. This is a rath, the Irish term for a ringfort, the type of enclosed farmstead that was built in its tens of thousands across Ireland during the early medieval period, roughly between the fifth and twelfth centuries. Most were the homes of farming families of middling status, the bank and external ditch serving as much as a boundary marker and a pen for livestock as any serious defensive structure.

The earthwork at Carrownaknockan measures approximately thirty-five metres across on its longest axis and twenty-nine metres on the shorter. The enclosing bank varies noticeably in width, running to around five and a half metres at the north and widening to seven metres at the south. Outside it runs a fosse, the formal term for a ditch dug to accompany an earthen bank, here roughly four and a half metres wide and nearly a metre deep at its maximum. Much of the western section of this fosse has been filled in over the centuries, and the northern stretch has disappeared almost entirely, the slow work of farming and weathering. The most likely position of the original entrance is on the north to north-east side, where the interior ground sits closest in level to the land outside, suggesting the builders left a low crossing point when the fosse was open.