Holy Trinity Abbey, Medieval Premonstratensian priory in County Roscommon, Ireland

Holy Trinity Abbey is a medieval monastery on Trinity Island in Lough Key, constructed from pale grey sandstone and limestone with a preserved west gable and pointed window. The building occupies the small island and displays typical features of medieval monastic architecture.

The monastery was founded in 1215 by Clarus Mag Máilin, archdeacon of Elphin, and received monks from Boyle Abbey in 1228. It gained abbey status in 1235, establishing itself as a monastic center.

The monastery's scriptorium produced the Annals of Loch Cé, a chronicle of Irish events from 1014 to 1590, written on Trinity Island. A 1466 entry in the annals blames a fire in the building north of the church on "the negligence of a woman", a detail confirmed by excavation. The island also holds the grave of Úna Bhán MacDermot, whose story is told by locals.

The archaeological site requires boat access to the island, as it covers about 0,9 hectares within Lough Key. Local operators offer guided tours to help visitors explore this remote location.

During an archaeological excavation in 1991, silver coins from the periods of Henry VIII and Elizabeth I were discovered on the monastery grounds. These coins suggest economic connections and trade with England.