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 width of the inner order is 1.60 m, considerably wider than the average Hiberno-Romanesque portal, indicating that one or more orders are missing. The inner order contains holes for bolts, suggesting that the original inner order (or orders) was removed in order to widen the door while the arch was in situ.

The simplicity of the chevron and the fact that the two orders duplicate each other suggests a date in the middle years of the 12thc.

A drawing of c. 1792 by Daniel Grose (National Library of Ireland 1976 TX 20) shows the simple (possibly 18thc) church that preceded the modern Church of Ireland building. At this time much of the S wall of the medieval building survived, running as far W as the Romanesque portal. A substantial buttress immediately to the N of the doorway demonstrates that this marks the position of the original W facade of the medieval church. While the arch may have been partially reconstructed, it thus lies in its original position.