The cathedral in Kildare town carries within its walls a surprisingly complicated history of collapse, abandonment, and reconstruction, to the point where the building a visitor sees today is partly medieval, partly seventeenth century, and partly a late-Victorian interpretation of what a medieval cathedral ought to look like. The northern transept, for instance, looks convincingly old; it was, in fact, entirely rebuilt after 1896, modelled closely on its southern counterpart. Even some of the floor conceals a small discovery worth knowing: twenty-one decorated orange medieval floor tiles, square and line-impressed, were found inside the cathedral, each just twelve centimetres across.

The site traces its origins to a pre-Norman church, making it one of the older continuously ecclesiastical sites in Leinster. By 1223 the building was already in a ruinous state, and it was at that point rebuilt by Ralph of Bristol, identified as the first English bishop of Kildare. The structure that emerged over the following centuries was a nave-and-chancel church in roughly coursed limestone, with a crossing tower and transepts; a crossing tower being one positioned at the junction of nave, chancel, and transepts, rising above the centre of the building on four piers with pointed arches opening in each direction. The stepped parapet at the top of the tower is attributed by the architectural historian Harold Leask to repairs recorded in 1395. Things went badly in 1588, when the roof was pulled down and the tower, chancel, and north transept subsequently collapsed, possibly because the foundations gave way. The choir was patched up again in 1686 and served in that reduced capacity until the major restoration begun in 1896, which rebuilt significant portions of the structure including the north transept, much of the chancel, the west windows of the nave, and three walls of the tower.

The exterior buttressing along the nave walls is one of the more arresting details to look for on a visit. Along both the north and south walls, six evenly spaced buttresses are joined at the top by pointed arches, forming a low arcade of six bays; five bays each frame a lancet window, a narrow pointed window type common in early Gothic architecture, while the remaining two bays frame opposing doorways. The effect is less like a cathedral wall and more like a covered passage laid against the building's flank. The main entrance is in the east wall of the south transept, a modern pointed doorway set within an earlier and larger pointed arch, above which a gable line hints at a projecting chapel that no longer exists.