Holy Trinity Priory was founded in 1215 by Clarus Mag Máilin, archdeacon of Elphin, the island being gifted by Cathal O'Reilly, king of East Bréifne.

[6] Excavation suggests that the building to the north of the church was rebuilt after burning down in the 15th century, and this is confirmed by an annal entry in 1466 which blamed a fire on "the negligence of a woman.

Holy Trinity Abbey was held in commendam by Ruaidri mac Diarmata (Rory MacDermot, King of Moylurg) from 1548 and granted to a Robert Harrison in 1594.

She was daughter of Brian Óg MacDermot (king of Moylurg 1603–36) who refused to let her to marry Tomás Láidir Costello ("Strong Thomas"), the son of one of his enemies.

[10][11][12] A 1991 excavation found silver coins of Henry VIII and Elizabeth I, bronze ferrule, iron knife and nails, pottery and rotary quern fragments.