The Castledermot monastic settlement was established by St. Dermot and recorded as the target of extensive Viking raids in 841 and 867. The monastic community itself ceased to exist sometime after 1073. The site at Castledermot today offers visitors the chance to observe the following sites of archaeological interest:
- The church, which features a reconstructed Romanesque doorway rising dramatically from the ruins.
- The North Cross, whose panels contain legible depictions of life scenes of religious figures such as King David, Isaac, and Jesus as he performed various miracles.
- The South Cross, with two of its panels clearly showing biblical scenes such as Christ being arrested, Daniel in the Lion’s Den, and Adam and Eve.
At the very edge of Castledermot stand the ruins of a Franciscan Friary estimated to have been founded in 1302 by Thomas, Lord of Ossong. The original building was long and rectangular in shape, with lancet windows and a tower. Additions were made to the building at later points in history. An interesting note – local history records that when one of the walls of the abandoned friary collapsed years ago, a hollow area was revealed that contained a number of human skeletons. The round tower is one of only two in Ireland that visitor can climb. The other is St. Canice's Round Tower in Kilkenny.