What survives is still legible enough: a roughly circular area about 26 metres across, enclosed by an earth and stone bank some 5.3 metres wide and standing to an internal height of 1.2 metres. It sits on a gentle north-facing slope in rocky pasture, with a natural rocky scarp running along its northern edge, a feature the original builders may well have incorporated deliberately into their sense of enclosure.

This is a rath, the most common type of ringfort found across Ireland, typically dating from the early medieval period, roughly the fifth to twelfth centuries. Raths were farmsteads rather than fortifications in any military sense; the enclosing bank defined a domestic space, protected livestock, and marked the household's social standing. At Carncash, the original entrance is still traceable as a 4-metre gap in the bank at the south-south-east, a conventional enough position for a rath entrance, oriented away from the prevailing weather off the north Atlantic. There is no fosse, the external ditch that often accompanies such banks, visible at ground level here, which suggests either that none was ever dug or that it has long since been filled and levelled. A smaller break in the bank to the north-east is a more recent addition, worn through by grazing animals rather than by any ancient hand.