This one is elaborately finished: a four-centred arch framing chamfered jambs and moulded bases, with a six-lobed marigold pattern carved around the drain-hole. It is a detail of considerable craft, still legible despite centuries of exposure, and it sits in a church that is otherwise in a fairly sorry state. The contrast between that small, precise ornament and the general ruin around it is what makes Roolagh quietly worth attention.

The church is a late medieval parish building, rectangular in plan and constructed from roughly coursed sandstone rubble. Three of its four walls still stand to full height, though the north wall has largely gone. The east gable retains a partially destroyed twin-light traceried window, the kind of decorative stonework associated with later medieval ecclesiastical building in Ireland. The west gable is an unusual feature: there are signs that this end of the building may have risen to two storeys, suggested by the remains of a now-destroyed window at first-floor level above a surviving single-light window at ground level. Along the south wall, a segmental-arched tomb-niche sits at the centre, a recess designed to hold an effigy or commemorative slab, and the doorway at the west end of the same wall was recorded in the Ordnance Survey Letters, as compiled by O'Flanagan in 1930, as having been a pointed sandstone door. The church sits on a natural rise of ground with views westward over the River Shannon, and a castle lies a short distance to the north. The graveyard that surrounds the ruin holds eighteenth- and nineteenth-century headstones, though recent works have removed the topsoil cover from much of the site.