Clarke's contribution includes a window depicting Rose of Lima, the sixteenth-century Peruvian mystic, an unexpected subject for a County Galway parish church. The interior also contains murals by Joshua Clarke and a depiction of Saint Grellan, a local early medieval saint associated with the territory of the Uí Maine, painted by Joseph Tierney. This accumulation of artistic work, distributed quietly across nave, aisles, and chancel, makes the building considerably more than its robust limestone exterior might suggest.

The church was commissioned in 1846 and designed by J. J. McCarthy, a Dublin architect whose practice became closely associated with the Gothic Revival style then fashionable for Catholic ecclesiastical buildings across Ireland. Construction ran from 1852 to 1858. The timing places it squarely in the period following Catholic Emancipation in 1829, when restrictions on Catholic worship and public life were lifted and the Church began investing heavily in large, conspicuous buildings as expressions of renewed institutional confidence. McCarthy's design draws on the vocabulary of Gothic architecture: lancet windows, pointed arches, a five-stage square tower topped with an octagonal spire, gargoyles at the tower corners, and cinquefoil and quatrefoil window tracery throughout. The cinquefoil and quatrefoil are decorative forms derived from the outline of five and four petals respectively, common in medieval ecclesiastical stonework and revived enthusiastically by Victorian Gothic designers. The limestone used throughout is carved with considerable care, in keeping with the character of many buildings in Ballinasloe, where the local stone lent itself to fine dressing and detail.

The church sits at the south end of Saint Michael's Square and is visible from a wide area of the town. The entrance from the square is marked by cast-iron double-leaf gates set between chamfered limestone piers with metal lanterns, with matching pedestrian gates to either side, a formal arrangement that signals the civic as much as the devotional ambition of the building.