The western half is the older structure, built in the early medieval period using cyclopean masonry, a technique in which very large, roughly dressed stones are laid together with minimal mortar, creating walls of considerable thickness, here around 0.85 metres. The same style appears at nearby Ardagh and Cloondara, suggesting a regional building tradition in this part of County Longford. Entry to this older section was through a lintelled doorway in the west gable, and windows once opened high in the north and south walls, though these have since been broken out. At some point the east window was blocked up entirely, and a new doorway cut through that same wall to connect with a later addition.

That addition, extending a further 8.3 metres to the east, dates to the late medieval period and almost certainly served as a chancel, the portion of a church reserved for the clergy and the altar. Its east gable shows a marked base-batter, meaning the wall splays outward slightly at its base for structural stability, and contains a single-light window from the 16th century. Inside the south wall, close to the east gable, are two aumbries, small recessed wall-cupboards used to store liturgical vessels, alongside a small flat-headed window. A relieving arch in the north wall of the chancel, designed to distribute the load above a now-blocked opening, hints at a doorway that was later sealed. The documentary record adds another layer: in 1593, a man named John Lye was granted a 21-year lease of the church, then referred to as the 'church of Agherie'. A few years later, an early 17th-century map of Shrule barony recorded it under a different name again, 'Lissevarrow', a reminder of how fluidly place names shifted in this period of significant upheaval in Irish landholding.