Built in the early 17th century, the castle was a typical Irish bawn; a defensive structure combining a fortified house with a walled courtyard designed to protect settlers during the Plantation period. The O’Malley family, who controlled much of Mayo at the time, originally constructed the castle before it passed to the Browne family, who would later become the Earls of Altamont.

The castle met its end during the turbulent 1650s when Cromwellian forces systematically destroyed fortifications across Ireland that might serve as strongholds for Catholic landowners. Following its demolition, the stones were repurposed for other buildings in the area, including parts of Westport House itself. Archaeological investigations have revealed the castle’s substantial foundations, showing it was a rectangular structure with corner towers and thick walls capable of withstanding siege warfare common to the period.

Today, visitors to Westport Demesne can still trace the outline of the castle’s foundations among the parkland’s mature trees and rolling lawns. The site offers a glimpse into Ireland’s complex plantation history, when fortified houses dotted the landscape as symbols of both colonial ambition and local resistance. Information panels near the ruins help visitors understand how this defensive structure fitted into the broader pattern of 17th-century Irish history, from plantation settlements through to the Cromwellian conquest that transformed Ireland’s political and social landscape forever.