That discrepancy in height, modest on the inside, considerable on the outside, is one of the subtler clues that this was once a deliberate defensive statement. A rath is an Early Medieval enclosure, typically dating from roughly the sixth to the twelfth century, defined by one or more earthen banks and ditches and used as a farmstead or high-status residence. This one at Shanacashel measures approximately 48 metres east to west and 44 metres north to south, making it a reasonably substantial example of the type.

The external fosse, a rock-cut or earthen ditch designed to enhance the height differential of the bank, still survives along the south-western to north-western arc, to a depth of around 0.8 metres. Its two ends are defined by later field boundaries, and a low, partially overgrown stone wall runs along its outer edge, suggesting that subsequent farmers found the old ditch a convenient place to anchor a division of land. The top of the bank has been further disturbed by cultivation ridges, the corrugated remains of lazy-bed farming that cross the interior on a roughly east to west axis, a reminder that the site was pressed into agricultural use at some point after its original function lapsed. A later field boundary has also been built directly atop the bank along its northern arc. The entrance, a gap 3.3 metres wide, faces to the east-south-east, a common orientation for raths across Ireland. Most strikingly, at the centre of the interior lies a souterrain, an underground stone-lined passage or chamber typically associated with storage or refuge, which here occupies the level heart of the enclosure while the area just inside the entrance dips noticeably downward toward the exterior.