This Anglo-Norman fortification was built in the 13th century during the initial conquest of Connacht, when families like the de Burgos were establishing their power across the western provinces. The castle would have been a typical motte and bailey construction; a timber palisade atop an artificial mound, designed to project Norman authority over the surrounding Gaelic territories.

By the late medieval period, Carn Castle had passed into the hands of local Irish lords who adapted it to their own purposes. The site became particularly significant during the turbulent 1590s, when it served as a stronghold for rebels during the Nine Years’ War. Its strategic position allowed defenders to monitor movement across this part of north Mayo, making it a thorn in the side of English forces attempting to pacify the region. The castle was eventually captured and dismantled by Crown forces in 1601, part of the systematic destruction of Irish fortifications that followed the Flight of the Earls.

Today, visitors to Carn will find only grass-covered earthworks and the faint outline of defensive ditches, but the site remains an evocative reminder of Mayo’s contested past. Archaeological surveys have revealed evidence of earlier settlement beneath the Norman layers, suggesting this spot held strategic importance long before the castle was built. Local tradition holds that a network of underground passages once connected Carn Castle to other strongholds in the area, though these tales remain unverified by modern excavation.