This substantial fortification, recorded in historical documents as MA030-056001, was built directly onto the bridge itself; a square tower that served as both a defensive structure and a toll collection point. The Reverend William Henry, writing in 1739, described it as 'a large square tower with a gate and guard room', suggesting it was still an impressive sight well into the 18th century.

The gatehouse at Abbeyhalfquarter would have been a familiar landmark to generations of travellers and merchants crossing the Moy. These bridge gatehouses were common features in medieval Irish towns, serving multiple purposes: they regulated trade, collected tolls, and provided a defensive checkpoint during times of conflict. The structure's position, rising directly from the bridge itself, would have made it impossible to enter Ballina from this direction without passing through its guarded gateway.

Today, a modern bridge carries traffic across the same stretch of the River Moy, with no visible trace of the medieval gatehouse that once dominated this crossing. The old structure likely met its end during the various road improvement schemes of the 19th and early 20th centuries, when many of Ireland's medieval bridges were widened or replaced entirely. While the physical gatehouse has vanished, its memory persists in the historical record, compiled by researchers like Jane O'Shaughnessy, reminding us of a time when crossing a river meant passing through stone walls and under the watchful eye of a gatekeeper.