The abbey at Inishmaine sits on a low promontory in a sheltered bay, the water having retreated or the land connection having been built up over time, and the stonework that remains is a concentrated record of early Christian devotion, Norman disruption, and medieval rebuilding.

The monastery here was founded by St. Cormac, and by the thirteenth century it had become a Benedictine cell, one of the small dependent houses attached to a larger Benedictine community. Maelísa, son of Turlough O'Connor, was prior of the abbey and died here in 1223; the site later functioned as a nunnery. In 1227, just four years after Maelísa's death, Richard de Burgo burnt Inishmaine, an episode that reflects the broader Anglo-Norman campaign to assert control over Connacht during that period. The early thirteenth-century church that survives has a nave and chancel, and the architecture repays close attention. The entrance to the nave is through a trabeate doorway in the north wall, meaning it is topped with a flat horizontal lintel rather than an arch, a form that may date to the early Christian period. The east gable holds a pair of round-headed windows in the Transitional style, the term used for Irish architecture that sits between the Romanesque and Gothic periods, set within a wide splayed embrasure with roll moulding and decorated hood moulding terminals. The chancel arch is incomplete but retains its base and columns with decorated capitals. North and south transepts were added later, both now badly damaged.

Northeast of the church stands a gate tower that appears on the 1838 Ordnance Survey map marked as a castle, and is thought to date from the fifteenth century. Its west wall is steeply battered, meaning it angles outward toward the base for structural strength, a feature common in late medieval tower construction. The main chamber had a vaulted ceiling and a stair accessed from the north; a twin-light window survives in the east wall of the first floor, though a number of other features visible today are modern reconstructions rather than original fabric. The site is a national monument in state care.