The pointed arch, carved from limestone and framed by crocket pinnacles, small decorative projections that curl upward like unfurling leaves, carries vine tendrils along its moulded surround, with birds caught mid-peck among bunches of grapes. The craftsmanship is of a quite different order from the building around it, and that is because the doorway is not original to the church at all. Cut into a horizontal slab just above the arch and beneath the hood-moulding, the date 1799 records the year it was placed here, transplanted wholesale into its new setting.

The doorway had spent roughly three centuries in another county altogether, in spirit if not in stone. It originated in the church of the Augustinian abbey of Fertagh, a monastic site located about three and a half kilometres to the north-north-east. When the doorway was removed from Fertagh in 1799 and reconstructed at Johnstown, as documented by Carrigan in his 1905 history, its medieval carvings came with it. Stylistically, the vine-leaf and bird decoration places it in the third quarter of the fifteenth century, a dating based on comparison with a doorway at the Augustinian Priory of St Mary's on Devenish Island, Co. Fermanagh. That parallel, noted by the architectural historian Harold Leask, suggests the carving belongs to a particular decorative tradition moving through Augustinian houses in Ireland during the late medieval period. A fragment of a similarly carved doorway survives at Ballygriffin church in Co. Tipperary, hinting at a shared workshop tradition or pattern book circulating among these communities. The east window now in the Johnstown church also came from Fertagh, meaning the building quietly holds two pieces of a dismantled medieval interior.