This is a cashel, the stone-built equivalent of the more familiar earthen ringfort, and the distinction matters here: where most Irish ringforts were defined by earthen banks and ditches, a cashel uses mortarless stone walling to mark out its territory. The wall at Carrickoneilleen stands around 1.3 metres high and 1.2 metres wide, enclosing a roughly circular area of 23 metres in diameter. There is no fosse, the defensive ditch that typically runs outside a ringfort's bank, visible at ground level, which gives the site an unusually clean, self-contained appearance from a distance.

A narrow break of about 0.9 metres in the northern section of the wall is thought to indicate where the original entrance once stood. Gaps of this kind are one of the more reliable clues archaeologists use when trying to read a monument that has no surviving documentary record, and here it is one of the few details that speaks directly to how the structure was once used rather than simply how it was built. Cashels of this type date broadly to the early medieval period in Ireland, roughly the sixth to twelfth centuries, when enclosed farmsteads of this kind served as the domestic and agricultural headquarters of individual farming families. The limestone rubble used in the construction is consistent with the local geology of County Sligo, where surface stone would have been readily available.

The site sits on a slight rise, which would have given its original inhabitants a modest but useful vantage over the surrounding land. Today, however, dense scrub growth has made the interior largely inaccessible, and the monument is more easily appreciated as a silhouette from the field margins than as something to be explored from within.