But the older church quietly complicates any straightforward narrative about the priory's origins. When Geoffrey De Marisco founded his monastery in 1183 in honour of the Blessed Virgin Mary, he had no Canons Regular to staff it, so he simply appointed the four priests already serving the church of St Kieran of Kells to act as canons until he could find others. The parish church, in other words, came first, and the monastery was built around its personnel.

The dedication to St Kieran, the fifth-century founder of the monastery at Saighir in County Offaly, was eventually set aside after the Norman Invasion, and both parish and church were rededicated to the Exaltation of the Holy Cross. The rectory was folded into the priory's holdings, a transfer confirmed by royal charter under Henry IV on 8 February in the thirteenth year of his reign. After the dissolution of the priory in 1540, the former prior, Nicholas Tobin, became curate at Kells, and for a time at least the priory nave seems to have served as the parish church. At some point, probably in the seventeenth century, the older church of St Kieran was brought back into use; it was certainly functioning again by 1668, when it appears in records relating to a union of parishes. Red brick alterations were made to the windows around that period, and a bellcote was added above the west gable, the wide rectangular structure with a pyramidal finial that still crowns the building today. The church continued in use as a Protestant place of worship until around 1850, when St Mary's was built about 150 metres away, and it was then abandoned.

What survives is a roofless shell roughly 26 metres long, with walls at the west end reaching nearly a metre in thickness, though Carrigan, writing in 1905, judged that the characteristic features had been so thoroughly altered as to make precise dating of the earlier fabric impossible. The stonework of the external west gable is largely rendered over, though cut-stone quoins are still visible at the south-west angle. More rewarding, for those who look closely, are two carved sandstone fragments set into the exterior jamb of the second north window from the west: chevron and lozenge decoration, probably salvaged from a doorway, chancel arch, or window of the pre-Norman building, worked into the later fabric as reused material. A fragment of an effigial tomb, the kind of carved stone monument typically marking a medieval grave, has been inserted into the external face of the graveyard's west wall at its southern end, a small and easy-to-miss detail that speaks to the accumulated layers of use this place has seen.