The ruined chancel of a medieval nave and chancel church, together with the partial remains of a residential tower attached to its north wall, survives in fragmentary form beside the later building. The two structures sit side by side in an unspoken argument about continuity and replacement, with a medieval font salvaged from the old church now housed inside the newer one. Wayside crosses stand within a few hundred metres to the north-east and south-south-west, and an Anglo-Norman motte and bailey castle, a raised earthwork fortification introduced by the Normans, lies roughly 480 metres to the south-south-west, giving the immediate landscape an unusual density of layered occupation.

The medieval church was, by the account of Sir Henry Piers writing in 1682, the largest parish church he had seen in the county, with a well-built steeple, a shingled roof, and a chancel raised nearly to the height of the nave. Alongside it stood what Piers described as a fair and handsome castle without a roof, a residential tower thought to have served as a priest's residence, built against the north-east wall of the chancel. The broader manor of Rathwire, within which the church sat, had deep Norman roots: the Annals of the Four Masters record King John receiving hostages there in 1210, the manor was formally granted to John D'Archy of Platten, Co. Meath in 1336, and by 1598 Rathwire was listed among the seven market towns of Westmeath. In 1606 the Earl of Clanrickarde purchased the surrounding lands, and following the Williamite Wars the estates passed to Robert Pakenham, who died in 1703 and is buried in the crypt beneath the present church. By 1801 the medieval fabric had deteriorated badly enough that the vestry resolved, with the Bishop's approval, to demolish it and build anew. The 1803 replacement was constructed partly over the nave of the earlier church. What survives to the east, the chancel wall with its three-light window with trefoil heads, a pointed arched doorway with dressed stone surround dating to the sixteenth or seventeenth century, a double-arched aumbry (a small recess typically used for storing sacred vessels) on the interior east wall, and the tower remains with the Farrell vault inside, represents the portion the builders left or could not easily clear. A wedge-shaped graveslab with a plain square plaque in relief is visible on the exterior of the south wall.