Trinity Abbey priory was founded in 1237, by Cathal O’Reilly, who granted Trinity Island to the Premonstratensian order "in pure and perpetual alms". It was a daughter house of the abbey of Trinity Island, Lough Key, Co. Roscommon. In 1427 Pope Martin V granted an "indulgence of three years and three quarantines of enjoyed penance" to all who would contribute to the repair of the "priory church of Holy Trinity, Lough Oughter", the penitents were to "devoutly visit the priory on the feast of the assumption". In 1567 the Lord Deputy of Ireland granted Trinity Island to Turlogh McCabe.

The west gable of the Abbey remains in good condition. It displays a pointed window over a mutilated doorway. Fragments of stone slabs with ornate and elaborate carvings are strewn about the adjoining graveyard

Now privately owned, the island is surrounded by 5km of lake shore walkways and is inhabited by a variety of local wild life. 

Trinity Island Lodge is available as a holiday home.