The five-light limestone window, perpendicular in style and dating to the 15th century, was not made for the 19th-century structure it now lights. It was salvaged from a Cistercian monastery that once stood on or very near the same ground at Inishlounaght, and set into the newer east wall where it still serves its original function, filling the chancel end with light.

The Cistercian abbey at Inishlounaght is no longer standing, but its presence persists in this repurposed stonework. The window is worked in limestone with punch tooling in vertical rows, a technique in which the surface is dressed by repeated small strikes to produce a regular, textured finish, and drafted margins, meaning the edges of the stone are cut back to a smooth border. Perpendicular Gothic, the style to which the window belongs, is characterised by its strong vertical tracery lines, and examples from 15th-century Irish monasteries tend to be somewhat plainer in execution than their English counterparts, though no less carefully made. The decision to incorporate the fragment into the later church rather than discard it was not unusual in 19th-century ecclesiastical building in Ireland; older monastic sites frequently provided both a location and, where fabric survived, ready-cut stone of considerable quality.