This rectangular limestone building, measuring just over 5 metres northwest to southeast and 4.3 metres across, features distinctive arched openings on opposite walls that once controlled access to the sacred grounds. The northeast arch spans 2.12 metres whilst its counterpart to the southwest measures 1.93 metres wide, both rising to an impressive height of 2.66 metres. These entrances retain fascinating defensive features including draw-bar slots; two at the northeast arch and one at the southwest, which would have secured heavy wooden doors against unwanted visitors.

The structure showcases typical medieval Irish building techniques, with walls constructed from uncoursed limestone rubble nearly a metre thick, whilst the arched doorways display more carefully dressed stone quoins. Beneath the southwest arch, traces of wicker-centring remain visible, offering a glimpse into the construction methods used by medieval builders who employed woven branches as temporary supports whilst the mortar set. Time and weather have taken their toll on the building; a window on the southeast wall has been broken out and the wall beneath subsequently rebuilt, whilst thick ivy now blankets the tops of the walls and what was once the roofed area.

From the gatehouse’s northeast corner, a substantial wall extends 8.6 metres along what was the boundary of the monastic enclosure, standing 1.6 metres high and helping visitors visualise the original extent of the abbey complex. Today, vertical metal posts have been installed in the southwest opening, a practical modern addition to prevent livestock from wandering into the abbey ruins. Despite these alterations and the passage of centuries, the gatehouse remains an evocative reminder of Canon Island’s ecclesiastical importance and the careful control once exercised over who could enter these holy grounds.