The presence of a souterrain, an artificially constructed underground chamber or tunnel typically built from dry-stone walling and used for cool storage or as a refuge, sets this ringfort apart from the more straightforward examples that survive across the Irish countryside.

The site at Kilmartin takes the form of a rath, the most common type of early medieval settlement in Ireland, in which a roughly circular area of land was enclosed by earthen banks for use as a family farmstead. Here the enclosure measures thirty-four metres in diameter and is defended not by a single bank but by two concentric earthen ramparts, reaching a maximum height of around two metres, with a fosse, or ditch, cut between them. A double-banked arrangement like this was generally considered a mark of higher status, suggesting the occupants commanded more labour and, possibly, more social standing than their single-banked neighbours. The entrance faces to the south-east, a common orientation in Irish ringforts, and the whole enclosure sits with an open view across the stream valley to the east, a position that would have combined practical drainage advantages with a clear line of sight over the surrounding land.