St. Mary's Cathedral (also known as Limerick Cathedral), is a cathedral of the Church of Ireland in Limerick city, Ireland. It is in the ecclesiastical province of Dublin. Previously the cathedral of the Diocese of Limerick, it is now one of three cathedrals in the United Dioceses of Limerick and Killaloe. The cathedral is open to the public throughout the week subject to church services.

St Mary's, dedicated to the Blessed Virgin Mary, was founded in 1168 and is the oldest building in Limerick which is in daily use. It has the only complete set of misericords left in Ireland.

In 1111, the Synod of Rathbrassil decided that "St. Mary's church" would become the cathedral church of the Diocese of Limerick. According to tradition, Donal Mor O'Brien, the last King of Munster founded the present cathedral on the site of his palace on King's Island in 1168. The palace had been built on the site of the Viking meeting place, or "Thingmote" - the Vikings' most westerly European stronghold. This had been the centre of government in the early medieval Viking city. Parts of the palace maybe be incorporated into the present structure of the cathedral, most prominently the great West Door, which tradition claims was the original main entrance to the royal palace. The West Door is now only used on ceremonial occasions. The Bishops of Limerick have for centuries knocked on this door and entered by it as part of their installation ceremony. According to tradition, during the many sieges of Limerick the defenders of the City used the stones around the West Door to sharpen their swords and arrows, and the marks they made in the stonework can be seen there today.

The tower of St. Mary's Cathedral was added in the 14th century, and it rises to 120 feet.

There are five chandeliers which hang from the ceiling. These are only lit on special occasions. The larger three of the five were made in Dublin and presented in 1759 by the Limerick Corporation. The belfry holds a peal of eight bells, six of which were presented by William Yorke, mayor of Limerick, in 1673. An active team of bell ringers travels the country to compete with other campanologists. St. Mary's received its organ in 1624, when Bishop Bernard Adams donated one. It has been rebuilt over the centuries, and was most recently renovated in 1968 and 2005.

During the Irish Confederacy wars, the cathedral briefly came back into Catholic hands. The Catholic Bishop, Richard Arthur, on his death in 1646, was buried in the cathedral.

In 1651, after Oliver Cromwell's forces captured Limerick, the Cathedral was used as a stable by the parliamentary army. This misuse was short lived, but was a similar fate to that suffered by some of the other great cathedrals during the Cromwellian campaign in Ireland. The troops also removed the Cathedral's original 13 ft Pre-Reformation high altar from the Cathedral. This altar was only reinstated in the 1960s. It is the largest such altar in Ireland and the UK, carved from a single limestone block.

In 1691, the Cathedral suffered considerable damage, particularly on the east end, during the Williamite Siege of Limerick. After the Treaty of Limerick, William granted £1,000 towards repairs. There are cannon balls from 1691 in the Glentworth Chapel inside.

In 1968, the Irish Government commissioned a postage stamp to commemorate the Cathedral's 800 year anniversary.

In 1991, there was a massive £2.5 million restoration programme that completed in 1996 with the excavation and re-laying of the floors, as well as the installation of underfloor central heating. Restoration continues today to a lesser degree.