In the east-north-east sector of that enclosure, low grass-covered wall-footings trace the outline of a structure that the 1837 edition of the Ordnance Survey six-inch map labels simply as a "Vault". The word suggests a burial vault, the kind of enclosed above- or below-ground chamber used to inter the remains of families or individuals of some standing, though the antiquity of this particular example remains uncertain.

The island itself has a long ecclesiastical history, associated with the early Irish church and its characteristic monastic settlements, where a surrounding enclosure wall, or cashel, would define sacred ground and separate it from the secular landscape. That the cartographers of the 1830s thought the structure worth naming and recording suggests it was still legible as a distinct feature at that time, even if its origins were already obscure. Whether it belongs to the medieval period of the monastery's active life or to a later phase of burial use on the island is something the surviving evidence does not resolve. What remains now is quieter still: a low ripple in the grass, following a rectangular plan, marking where walls once stood.