The ruined cathedral consists of an aisleless nave and chancel, to which a sacristy was added on the S. The nave was originally a single cell, rectangular church, with antae, belonging to the pre-Romanesque era. Its internal dimensions are approximately 14.7 m by 9.0 m, making it one of the largest such buildings in the country (along with Clonfert and Clonmacnoise). There is an early lintelled doorway in the W wall. The chancel, which has internal dimensions of 6.63 m by 11.45 m, was added about 1200, when an additional doorway was inserted in the N wall of the nave. The chancel is built of rubble masonry, but there are well-dressed quoins on the E angles, which are given a tiny chamfer.

Soon after the Anglo-Norman invasion, attempts were made to unite the diocese of Glendalough with that of Dublin. The Anglo-Normans mounted a propaganda campaign against the old monastery, since it lay in an area, which the new regime found hard to control. In a document of 1215 it was described as a 'nest of thieves and robbers'. The remodelling of the cathedral took place while the independence of the see of Glendalough was under question, and it is tempting to see the alterations as a belated attempt to raise the status and appearance of the cathedral church. The union of the dioceses finally received Papal approval in 1216, after which Glendalough was reduced to the status of an archdeaconry. The cathedral appears to have become an Augustinian house and in the 1302-6 it was described as the 'Priory of the great church of Gleydelagh', presumably to distinguish it from St Saviour's. Before passing into the care of the Office of Public Works, the cathedral fabric was in a far more ruined condition than it is today. Both the chancel arch and the E window were substantially rebuilt between 1875 and 1879.

There is a drawing in the Royal Irish Academy (3C. 30) by the 18thc topographical artist, Gabriel Beranger, which purports to show the capitals of the rear arch of the E window. One side shows various floral designs and two standing affronted animals; the other shows what appear to be a lion attacking a snake, followed by a human figure grasping foliate tendrils. The drawing was later copied by Petrie.

The early Gothic character of the mouldings of the window and the N door of the nave suggest that the alterations to the cathedral could not have taken place much before 1200, in other words not long before the diocese was united with Dublin. During the alterations limestone imported from Dundry Hill, near Bristol, was employed for the dressed masonry, a material favoured by masons working for the Anglo-Normans.