It looks, at first glance, like a straightforward example of early nineteenth-century ecclesiastical building. What makes it more interesting is what may be buried within its walls. Ordnance Survey correspondence recorded that the church was built close to a structure known as Church Castle, which may itself have been the surviving tower of a far older St Mary's. The traveller Richard Pococke, passing through in 1752, described seeing a church built directly onto the tower of an earlier one, and that same tower, along with two ruined buildings beside it, was later illustrated in Francis Grose's Antiquities of Ireland in the 1790s. It is quite possible that the 1820 church absorbed some of this medieval fabric into its own fabric, though the matter remains unresolved.

The site's history stretches back at least to the late thirteenth century, when St Mary's church came under the control of Owney Abbey, a Cistercian house in County Limerick. The church is mentioned by name in 1432, and the connection with Owney Abbey continued until 1562, when the abbey's possessions were transferred into lay hands following the dissolution of the monasteries. By 1656, when the Civil Survey was compiled, the building was already recorded as a ruin, though the graveyard continued in use; in 1673, Lady Thurles was buried in what was then called Ladyes Chapple, beside the old St Mary's. Whether this site represents the original medieval parish church is itself uncertain, since it lies outside what is understood to have been the medieval core of the town.

The graveyard holds several objects worth close attention. The most remarkable is a table-tomb dated 1520, commemorating the Archer family of Archer's Court, which Pococke also noted during his visit. A table-tomb is a raised altar-like structure, and this one carries double effigies of a knight and his lady on its upper surface, their feet resting on a carved dog, a common medieval convention symbolising fidelity. Around the sides, six apostles are set into round-arched niches supported by moulded columns, and one end bears a carved Crucifixion scene with the figures of Our Lady and St John. Elsewhere in the graveyard, a wall memorial of 1683 displays a coat of arms topped by a helmet with a lion rampant standing upon it, the initials IG and EP inscribed on a scroll below; it rests on a limestone altar set against a free-standing stone wall. On the south side of the church, a recumbent graveslab from the sixteenth or seventeenth century carries an eight-armed floriated cross, its lower section decorated with symbols of the Passion.