Every other medieval parish church in the city is gone, absorbed into later fabric or simply lost. This one endures, and it does so in a quietly complicated way, because the building is not really one thing but several centuries of accumulation pressed together, some parts roofed and in regular use, others open to the sky, and the whole place threaded with monuments, floor slabs, and architectural details that most visitors walk past without realising quite how old they are.

The site itself may go back further than the Norman church. It stands on what is thought to have been the location of an earlier church associated with St Columba, and a cross-slab dating from the late tenth or early eleventh century has been recorded here. The dedication is to St Ouen, Bishop of Rouen, who died in 684, and the church was granted to the Augustinian Convent of Gracedieu around 1190, before passing to the Treasurer of St Patrick's Cathedral in 1219. The St Anne's Chapel in the south aisle was established in 1431 for the Guild of the Fraternity of St Anne, a trade guild association; a chantry chapel, which is essentially a privately endowed chapel set aside for prayers for the souls of its founders, was added to the south side of the chancel in 1482 by Roland FitzEustace. FitzEustace, who died in 1496 as Lord Portlester, is commemorated inside along with his wife Margaret Jenco by a large granite slab depicting two recumbent figures, a knight in armour and his lady. The west tower, built of coursed limestone and rising to just over twenty-seven metres, partially collapsed in 1669 and was restored in the 1670s; three of its six bells, however, date from 1423. A font found beneath the nave floor in 1848 carries the date 1192 incised in Arabic numerals on its side, alongside shell designs carved in shallow relief.

The church sits on Cornmarket, approached from the south along High Street or from the lane that runs beside the old city wall. The nave remains an active Church of Ireland parish church, entered through a partially reconstructed twelfth-century round-headed doorway. The roofless chancel, notably not aligned with the nave but inclined to the northeast, and the re-roofed St Anne's Chapel, now used as an exhibition space, are accessible separately. Look for the corbels still projecting above the nave piers, which once carried a timber roof, and for the graveslabs set into the floor. The Portlester monument and two other medieval memorials have been moved into the tower for protection. Excavations in 1992 uncovered the remains of what may have been a domestic range to the south of the church, suggesting the site once supported a more extensive complex than the standing fabric alone implies.