This is a rath, the Irish term for a ringfort, one of tens of thousands of such enclosures that once served as the defended homesteads of farming families across early medieval Ireland. Most were built roughly between the fifth and twelfth centuries, and while the grander examples survive as dramatic earthen rings, many others have been worn down to faint suggestions, legible mainly to those who know what to look for.

The enclosure at An Muirneach Beag measures approximately 27 metres east to west and 24 metres north to south, making it a modest but reasonably typical example of the type. What defined it originally was an earthen bank, the interior height of which still reaches 1.2 metres at its best-preserved section running from the north-west towards the south-east. On the southern to north-western arc, the enclosure is marked instead by a scarp, a slope or cut in the ground roughly a metre high, where the bank material has either eroded or been removed. Adding a further layer of time to the site, a stone field boundary has been built directly on top of part of the surviving earthwork, a common enough fate for ringforts in agricultural landscapes, where later generations of farmers found ready-made ridges convenient for dividing land without fully understanding, or particularly caring, what lay beneath.