The fort at Corramegan measures about 29.5 metres north to south and 28 metres east to west, defined not by the kind of earthen bank and ditch arrangement typical of many Irish ringforts, but by a scarp, a slope cut into or shaped from the natural ground, running from the south-east around to the north-north-west. There is no visible fosse, the defensive ditch that usually accompanies such a feature, which makes the site slightly harder to read in the landscape than a more conventional enclosure.

The most quietly compelling detail lies just north-west of centre, where a depression in the ground traces an L-shaped path, running roughly east to west before turning north. It measures around eight metres along its longer arm and reaches a depth of about 0.9 metres. This is thought to be the remains of a souterrain, an underground passage or chamber built during the early medieval period, typically used for storage or as a place of refuge. Souterrains were often constructed in dry-stone or earth-cut form and roofed with lintels; when they collapse or are robbed of their stone over centuries, they leave behind exactly this kind of surface depression. The feature at Corramegan extends toward a break in the bank at the north-west, suggesting that was once a functional opening, possibly the original access point into the structure below, if not into the enclosure itself. By 1968 the site was relatively clear of vegetation, but the perimeter has since grown dense with furze, which has gradually obscured the scarp and made the full outline of the fort harder to trace from ground level.