Illauntannig, the largest of the Magharee Islands, sits in the waters between Brandon Bay and Tralee Bay, and what survives on it is an extraordinary concentration of early medieval remains: two small oratories, three beehive huts, a souterrain (an underground stone-lined passage, typically used for storage or refuge), three leachts (low rectangular cairns associated with prayer or commemoration), a burial ground, a stone cross, cross-slabs, a hand-bell, and fragments of five quern-stones. A bullaun stone, a large rock with one or more cup-shaped hollows often linked to early Christian ritual use, sits within the enclosure, and a second lies roughly a hundred metres to the south, at the edge of the sea. The whole settlement is enclosed by a cashel wall, a drystone boundary typical of Early Christian ecclesiastical sites in the west of Ireland.

The cashel itself has a complicated history that is only partially legible. It encloses a roughly oval area about 51 metres north to south and 43 metres across, and on the western half the wall survives between 4.5 and 5.5 metres thick. The eastern half has suffered badly from coastal erosion, and a concrete retaining wall now stands in for what the sea has taken. When the antiquarian Lord Dunraven surveyed the site in 1875, he described it as considerably destroyed and could trace the outer face for only about six metres. Earlier still, an Ordnance Survey notebook from the early nineteenth century recorded the wall as barely rising above ground level. Yet the remains visible today are substantially more impressive than either account suggests, which points to deliberate intervention at some point, whether the clearing of accumulated rubble, partial rebuilding, or both. A drystone retaining wall not recorded on Dunraven's 1875 plan may represent a late nineteenth or early twentieth century conservation attempt. There is even a second outer wall face on the western arc whose chronological relationship to the main wall remains unresolved. A 5.5-metre-long entrance passage on the west-southwest side gives access to the interior, and just south of it the inner wall-face forms a small D-shaped recess whose purpose has not been established.

The island is accessible by boat from the Maharees peninsula, and the crossing, the exposure, and the scale of what awaits on the other side make for an unusual combination of effort and reward. Reennafardarrig, a headland on the adjacent mainland, may also be related to the early settlement there, with a hut-site, old field walls, and a boulder reportedly inscribed with a cross all noted in the vicinity.