The years 1766 and 1809 are cut into the quoins, the dressed corner stones, of the building: one on the north-west corner of the nave, the other on the south-west corner of the south transept. It is an unusual form of self-documentation, the building annotating its own construction history in the fabric of its walls.

The church is known as St Finbar's South, sometimes called the South Chapel, and it takes a cruciform plan, meaning it is laid out in the shape of a cross, with a nave, chancel, and transepts extending to either side. The original structure, nave, chancel, and possibly a north transept, was built in 1766. The south transept followed in 1809, expanding the building along its Latin-cross axis. The west gable, which faces onto the street, shows considerable evidence of rebuilding in its central section, and the door surround is modern. Inside, the picture is less encouraging. The architectural historian Maurice Craig, writing in 1982, described the interior as having been "brutally gutted", a candid verdict on the modernisation that stripped much of whatever original character it once held. One significant exception survives at the altar: a sculpture of the Dead Christ by John Hogan, the Cork-born neoclassical sculptor who spent much of his career in Rome and whose work appears in several Irish Catholic churches from the mid-nineteenth century. It is a quietly affecting piece in a space that otherwise gives little indication of its eighteenth-century origins.