Castledermot was an important Early Christian monastery, with a round tower and two well-preserved high crosses. There is a 19thc Protestant church on the site (incorporating some Gothic features), to the W of which a Romanesque arch has been preserved as a folly. The design of the 12thc arch was used as a model for the main doorway of the modern church.
The jambs hava a single angle roll, identical to the first order. There are no traces of decoration on the capital. The arch, of 20 voussoirs (one is a modern replacement), has frontal chevron on the face, as on the first order.
The jambs hava a single angle roll. The capitals are too worn to discern any ornament; there appear to be traces of diagonal patterns on the inner face of the N capital. The arch, of 17 voussoirs is carved with chevron frontal to the face. The springers have centering holes.
H. G. Leask, Irish Churches and Monastic Buildings, Dundalk, 1955, 162.