St. Mary's Abbey, Duleek, Medieval monastery ruins in Duleek, Ireland.

St. Mary's Abbey in Duleek is a monastery that dates to the medieval period in Ireland. The ruins display stone walls, cloisters, and architectural features characteristic of 12th-century Romanesque construction, allowing visitors to understand the building's original scale and design.

A monastery stood at Duleek from the 5th century, and its round tower was damaged by lightning in 1147. In 1180 Hugh de Lacy, Lord of Meath, granted Duleek and St. Patrick's Church to the Canons Regular. The surviving south aisle and tower are later work, and the abbey was shut down in the 1537 Dissolution of the Monasteries.

The abbey was suppressed in the 1537 Dissolution of the Monasteries, after which the site was used for burials. James Cusack, Catholic Bishop of Meath from 1679 to 1688, lies in an effigial tomb here, as does John Bellew, 1st Baron Bellew of Duleek, who died of wounds received at the Battle of Aughrim, and his son Richard, 3rd Baron. Saint Kienan's Church of Ireland church was built beside the ruins in 1816.

The site is accessible during daylight hours and can be explored on foot, with the ruins in varying states of preservation. Sturdy footwear is recommended as the ground can be uneven, and a visit during the middle of the day provides the best light for viewing the stonework.

In the north wall of the medieval belfry, a scar or shadow marks where a round tower once stood; that tower was struck by lightning in 1147 but survived for several more centuries. The east window is a 1587 post-Gothic replacement, with the arms of Sir John Bellew and Dame Ismay Nugent carved beneath it.