Set into the fabric of a later building, it is a Romanesque doorway of three decorated orders, meaning it has three concentric arched bands of carved stonework receding inward around the opening, a form typical of twelfth-century ecclesiastical architecture in Ireland. What makes it unusual is that nobody is entirely certain where it came from, and the question has never been fully resolved.

The traditional account, recorded by an Urban Survey in 1989, holds that the doorway was brought from Old Kilcullen, a site in County Kildare associated with early Christian remains, and re-assembled as the south entrance to the church. The stone is described as a slaty blue limestone, and whoever reassembled it incorporated fragments of a later door, probably thirteenth-century, into the base, suggesting the reconstruction was not entirely tidy or perhaps drew on whatever salvaged material was to hand. A competing and arguably more plausible explanation is that the doorway never travelled far at all. Archaeologist Chris Corlett has suggested it more likely originated from a Romanesque church that may have stood within or very close to the graveyard of the present church itself, which would mean the doorway was reused from an earlier building on roughly the same site rather than transported across county boundaries.

The doorway is visible as the south entrance to the church, and the carved stonework of the three orders, alongside the inserted fragments at the base, rewards a close look for anyone interested in how medieval stonework was recycled and adapted across centuries of building and rebuilding.