Yet this is, by some measures, the most architecturally sophisticated of Glendalough's medieval buildings, and one of the latest in the sequence. Its chancel arch, built to three orders, carries Romanesque carved ornament, the kind of interlaced and figurative decoration that was reaching its most elaborate development in Irish stone-carving during the twelfth century, and the two-light east window repeats the same vocabulary of human and animal figures in carved stone.

The church at Derrybawn is said to have been founded in 1162 by St Laurence O'Toole, the Archbishop of Dublin whose reforming ambitions shaped several Irish monasteries of the period. He established it for Augustinian Canons, a congregation of clergy who followed the Rule of St Augustine and lived a communal life broadly resembling that of monks. Shortly after 1163 the house became Arroasian, meaning it affiliated with the reformed congregation based at the Abbey of Arrouaise in northern France, a stricter observance that had spread rapidly through Ireland in the preceding decades. The buildings, now largely restored, comprise a nave measuring roughly 12.7 metres by 6.35 metres and a smaller chancel, with a possible domestic range attached to the north side of the nave. The chancel would originally have been covered by a barrel vault, a curved stone ceiling common in Romanesque construction, and the east wall retains two aumbries, small recessed cupboards used to store liturgical vessels, one on either side of the window. A mural stair built into the wall of the attached range once led to an upper floor, suggesting the building served practical as well as devotional purposes.

The church sits on the south bank of the river and can be reached on foot from the main Glendalough monastic enclosure. It is worth pausing at the chancel arch once inside; the carved figures are modest in scale but unusually detailed for a church of this size, and the arrangement of the east window and its flanking aumbries gives a clear sense of how the liturgical space was organised for a small community of canons.