It is on a complex site archeologically, and the ground level was raised substantially before it was built using re-used stone around the year 400. There is evidence of restoration using brick in subsequent centuries. What survives is part of the apse and a stub of wall to the east. Many confused remains of burials were found around the building and below the floor, and the most interesting of these is a sarcophagus with strigillate decoration and with a relief of the Good Shepherd, which has an inscription on the lid "Quiriacus dormit in pace" (Quiriacus sleeps in peace). He was a bishop of Ostia who was martyred in the town in the year 269 with seventeen converted soldiers, and it seems almost certain that this sarcophagus was re-used to hold his relics when they were enshrined here. The church was recorded as still in use as a pilgrimage site in 1162, by which time it was probably the only intact building in the entire ancient town. However, it fell into ruin and was forgotten at an unknown date. The relics of the saint were taken to a nunnery in the Corso, and when that failed in the late middle ages they were finally enshrined in Santa Maria in Via Lata.
The sarcophagus has been left in situ, but the inscription is in store for safe keeping.
Ostia, Oratorio di San Quiriaco is a small church just south-east of the theatre in the ruins of ancient Ostia.
http://www.cyriac-fhp.com Cyriac (Quiriaco et al, 2,600+ spellings) Family History Project