One of the great churches of the 12th century, St Mary's Abbey, was built by the Augustinians on lands presented to them by Hugh de Lacy, Overlord of Meath and surrounding areas. In the 16th century a massive square tower was built alongside the earlier round tower. The latter is no longer standing but the scar where it was joined onto the square tower is clearly visible on its northern face.

In the churchyard are two Celtic high crosses -The North Cross and The South Cross. These are believed to date from the 10th century. They are similar to The Great Cross of Monasterboice. Within the church are some early cross-slabs, a Romanesque pilaster-capital and the base and head of the South Cross. In the aisle are some 16th and 17th century monuments including an effigial tomb of Dr. Cusack, Bishop of Meath 1679-88. Also here are two tombs - one covers the remains of Lord Bellew who was killed in action at the Battle of Aughrim 1691 and a mensa-slab is supported by tomb-surrounds bearing the arms of Bellew, Plunkett, Preston and St Lawrence. The east window (bearing the arms of Sir John Bellew and Dame Ismay Nugent beneath it) is a 1587 post-Gothic replacement. The remains of this abbey are joined in the grounds by the now disused St.Cianan's Church of Ireland church which was built in 1816. Interred in the graveyard are the remains of Catholic, Church of Ireland and Huguenot believers. The ruins of St Cianan's original stone church, built in 489, are to the northwest of St Mary's Abbey and across the road from it.