St. James’s Church lies on the site of a monastery founded around 800 by the father of St.Diarmuid, after which Castledermot takes its name. The monastery was raided by the Vikings in the 9th century, but continued its existence at least until the 12th century. All that is left today is a splendidly reconstructed Romanesque doorway, which came from a church that has since vanished, a 10th-century Round Tower, 66ft high with granite base, and two magnificent High Crosses, probably 9th century.
The round tower stands on the north side of St. James church, to which it is attached by an ancient narrow and high passage 8’ in length. The masonry consists of rough unworked granite boulders. The spaces between them are filled up with common quarry stores embedded in the mortar. The height of the tower is 66.5’ and the walls at the base are 3.5’and are inclined upwards.
Richly carved with depictions of the Crucifixion, Adam and Eve, Daniel in the Lions’ Den, the Sacrifice of Isaac, these High Crosses are among the best preserved of the granite crosses in the Barrow valley. The North Cross shows David with his harp, one of the few images from this time of an Irish harp. The south cross's east face is richly decorated with abstract Celtic design.
Also in the churchyard are the foundations of a medieval church and a number of well preserved early Christian and medieval grave slabs.