Sorted by: Former or intended Cathedrals
Annaghdown Cathedral Ruins, Co. Galway
The first monastery was founded by St Brendan of Clonfert in 6th century. Several 'bishops of Annaghdown', from 1189 to 1485, were systematically elected by its 'Cathedral Chapter' and, despite many counterclaims from Tuam, some were approved by Rome. A 15th century building of Annaghdown Cathedral has an inserted east window of the late 12th century. It is a round-headed window with a richly decorated Romanesque frame.
Ardfert Cathedral Ruins, Co. Kerry
It was built between the 12th and 17th centuries on the site of St Brendan The Navigator's 6th-century monastery. The ruin retains impressive Romanesque west doorway, the 13th-century east window, and a beautiful row of 9 lancet windows in the south wall. Also here are two smaller churches.
Ardmore Cathedral and Round Tower, Co. Waterford
It is believed to be the oldest Christian settlement in Ireland. St. Declan lived in the region at some time in the period 350-450 AD and christianised the area before the coming of St. Patrick. There is a well-preserved 12th-century Round Tower and the ruins of a Cathedral and oratory dating from the 13th and 8th centuries respectively.
Clonmacnoise was founded in 545 by Ciaran (Kieran) of Clonmacnois, one of the 12 Apostles of Ireland. The site includes the ruins of a cathedral, 7 churches (10th-13th century), 2 Round Towers, 3 High Crosses, the largest collection of Early Christian graveslabs and a 13th century castle ruins.
Glendalough Monastic Site, Round Tower and Valley, Co. Wicklow
It was founded by St. Kevin in the 6th century. Set in a glaciated valley with two lakes, the remains include a superb round tower, stone churches and decorated crosses. The wooded valley contains stunning scenery, way-marked walking trails, if you are lucky you will see some wild deer and goats.
Kilfenora Cathedral of St. Fachtna and High Crosses, Co. Clare
Kilfenora's main claim to fame is its ruined cathedral and associated high crosses. At one time Kilfenora became a small diocese in its own right (c.12th century). The cathedral has a variety of carved figures, tombs and windows.
Kilmacduagh Monastic Site and Round Tower, Co. Galway
This interesting early Christian site is well worth a visit for its ruined churches and well preserved round tower the highest surviving in Ireland (and leaning 0.5m out of perpendicular). The monastery was founded here in the 7th Century by St. Colman MacDuagh.
A group of medieval buildings set on an outcrop of limestone in the Golden Vale including the 12th century Round Tower, High Cross and Romanesque Chapel, 13th century Gothic cathedral, 15th century Castle.