The bank that defines it is so overgrown as to be easily mistaken for a natural feature, and the external fosse, a defensive ditch that would once have reinforced the boundary, is now only detectable as a band of unusually lush vegetation some five metres wide running around the outside.

This is a ringfort, or at least a structure of that type, the kind of enclosed circular or near-circular earthwork that appears in enormous numbers across Ireland, most dating to the early medieval period between roughly the fifth and twelfth centuries. They functioned primarily as defended farmsteads, the bank and fosse combination discouraging cattle raiders rather than armies. At Cornaglah, the bank survives to an internal height of just 0.25 metres and an external height of 0.5 metres, meaning the outer face still presents a slightly more imposing profile than the interior suggests. A western entrance is visible, though whether this reflects the original access point or a later break in the earthwork is not established. The interior dimensions, at 31 metres east to west, sit comfortably within the typical range for such enclosures.