Located in low-lying ground beside a river, this castle first appears on a 1563 map of Leix and Offaly, marking it as an important enough fortification to warrant cartographic attention during the Tudor period. The castle operated as a dependency of the more prominent Dunamase fortress, suggesting it likely served as an outlying defensive position or administrative centre within a broader network of Anglo-Norman strongholds in the region.

The castle’s complete disappearance speaks to the transformative changes that swept across the Irish landscape during the industrial era. When engineers plotted the route for the Grand Canal in the late 18th century, they drove it straight through where Ballymanus Castle once stood. Various editions of the Ordnance Survey 6-inch maps clearly show the canal cutting across the former castle site, a testament to how Ireland’s industrial ambitions quite literally overwrote its medieval heritage. The canal builders either demolished whatever ruins remained or incorporated the stone into their new waterway infrastructure; a common practice that saw countless historical structures recycled into bridges, locks, and embankments.

Today, visitors to Derrybrock will find only the peaceful waters of the Grand Canal where Norman knights once kept watch. The castle’s story survives primarily through historical maps and archaeological records, preserved in works like the Archaeological Inventory of County Laois. Whilst the physical castle has vanished, its documented existence helps piece together the complex patterns of power, settlement, and defence that shaped medieval Laois, reminding us that even absent monuments can tell compelling stories about Ireland’s layered history.