What makes it unusual is the layered complexity of its defences: a subcircular inner wall of mortared masonry, roughly 35 metres north to south and 30 metres east to west, sits within a deep wide fosse, a defensive ditch, which is itself enclosed by a low outer bank known as a counterscarp. This arrangement, concentric and deliberate, belongs to a tradition of serious fortification rather than simple settlement.

Radiocarbon dating of mortar from the inner wall points to a construction date somewhere between AD 1020 and 1180, placing the structure firmly in the pre-Norman period. Researchers Naessens and O'Conor have identified it as a possible Irish fortification of the 11th or 12th century, built before the Anglo-Norman arrival that would reshape so much of the Irish landscape. That context matters, because just 450 metres to the north-east lie the remains of Cargin Castle, a 13th-century structure representing the new order that followed. The two monuments, separated by a short stretch of water, effectively bracket a political and architectural transition. A narrow opening in the cashel-like inner wall and the remnants of two internal wall projections appear to be later additions, likely dating to the late 18th or early 19th century, when the island was home to a recluse named Carbry, the man from whom Illauncarbry takes its present name. The island has since been entirely planted with hardwoods, which have now reached maturity, giving the whole enclosure an atmosphere of deep enclosure and quiet seclusion that has little to do with the medieval world that built it.