It ceased to function as a cathedral in 1214, when the diocese of Glendalough was absorbed into the diocese of Dublin, leaving the largest structure in the entire monastic complex technically redundant in ecclesiastical terms for the better part of eight centuries. It sits on a small plateau in the south-eastern sector of the main enclosure, overlooking the junction of the Glendasan and Glenealo rivers, dedicated originally to Saints Peter and Paul, and built not in one campaign but in several, each generation leaving its own legible trace in the stonework.

The earliest fabric survives in the nave walls as cyclopean masonry, large irregular stones fitted together without mortar in a technique that signals genuine antiquity. Rebuilding followed in the tenth century, and the chancel was added towards the end of the twelfth. The chancel, notably, is not quite in line with the nave and is of inferior masonry, small discrepancies that hint at different builders working at different moments without perfect coordination. Most of the stone is local mica schist, but some of the later decorative elements use Dundry stone, a limestone quarried near Bristol and shipped across the Irish Sea, which suggests ambition and external connection. The chancel arch is of granite carved with chevrons, and the west doorway has the inclined jambs characteristic of early Irish Romanesque work. Some reconstruction was carried out in the 1870s, though the bones of the medieval structure remain. Inside, five cross-slabs rest against the north wall of the chancel. One carries two Old Irish inscriptions, prayers for individuals named Maccois and Diarmait, while another commemorates Muirchertach Ua Cathalan, a notable figure killed at the battle of Moin-mor in 1151. These formulaic inscriptions, beginning "Or do", meaning "a prayer for", are a common feature of early Irish memorial stones, but the specificity of Muirchertach's slab, tying it to a datable battle, makes it an unusually precise historical document in stone. Around the cathedral, some 76 further undecorated slabs have been counted, along with various crosses and cross-bases. A tub-shaped cross-base or font sits just inside the west door. And on the old six-inch Ordnance Survey map, a spot to the west is marked as the site of St Kevin's Yew Tree, though no other record of it appears to survive.