The large east window now set into the church's east gable was lifted from an altogether different structure, roughly three and a half kilometres to the north-north-east, and rebuilt in its current home in 1799. It is the kind of quiet transplantation that happened often enough in the eighteenth and nineteenth centuries, when older ecclesiastical stonework was considered fair salvage, yet it still produces a mild disorientation when you know what you are looking at.

The window came from the church of the Augustinian abbey of Fertagh, a medieval foundation whose fabric was clearly being dismantled or at least raided by the close of the eighteenth century. Along with the window, a west doorway was also removed from Fertagh at the same time and rebuilt in the Johnstown church. The east window itself is a substantial piece of work in cut limestone, chamfered on the exterior, with four round-headed lights gathered under flowing tracery within an overall pointed frame. Flowing tracery, which uses curved, branching stone bars to fill the upper portion of a window, was a Gothic development that allowed for more elaborate and organic-looking compositions than the earlier geometric styles. Here, though, the tracery is noted as somewhat unbalanced, lending it what one might charitably call an idiosyncratic quality. The hood-moulding, the projecting frame that runs over the window to deflect rainwater, ends in horizontal stops decorated with carved foliage: vine leaves on the right-hand side and ivy leaves on the left, a detail that mirrors the treatment found on a second Fertagh window, which ended up not here but in the local Roman Catholic church.