Abbey of St. Mary de Portu Patrum, Annaghdown, Romanesque abbey ruins in Annaghdown, Ireland.

The Abbey of St Mary de Portu Patrum, also called Annaghdown Priory, is a ruined house of Arroasian canons in the townland of Annaghdown, County Galway. Its remains comprise a church, a cloister and living quarters. Window details point to 13th-century construction, while the south window of the choir is in the transitional style of about 1200.

The monastery was founded in 1140 by King Turlough O'Conor for Arroasian canons and operated until its closure during the Reformation around 1562. This span of over 400 years demonstrates how the site remained important through different periods of Irish history.

The carved stone capitals in the cloister show animals and plants carved into local limestone, reflecting the artistic skills of medieval craftspeople. These decorations reveal how monks expressed their faith through everyday details of their living spaces.

Annaghdown Cathedral lies about 110 metres southeast of the abbey ruins, so the two monuments can be seen on one walk. The ground around the ruins is uneven, and there are no facilities on site, so wear sturdy footwear and bring water. Daylight is best for reading the carved windows.

The east window of the abbey church is missing, and Annaghdown Cathedral, 110 metres to the south-east, has an east window of similar date. Author Peter Harbison argues that its perfect fit makes a late move unlikely, so the abbey glass may never have travelled.