The castle’s origins date to the 1240s when Maurice Fitzgerald, an Anglo-Norman baron of the Kildare Geraldines, constructed a fortress to control this strategic crossing point. Built on a small rise overlooking the harbour, bridge, and town in what’s now Quay Street, the castle commanded the vital crossing over the River Garvoge. Fitzgerald likely replaced the existing wooden bridge with stone, creating an interconnected defensive system that secured both river access and the roads connecting upper Connacht to the sea. From this stronghold, he launched attacks northward into O’Donnell territory in Tír Conaill.

The castle changed hands repeatedly throughout its tumultuous history, passing between Anglo-Norman families before Richard de Burgo acquired both castle and manor in 1299. De Burgo substantially rebuilt the structure, possibly incorporating remnants of Fitzgerald’s original construction. By the late 1300s, the Gaelic O’Connor family had emerged as keepers of the castle, maintaining impressive fortifications that caught the attention of Sir Henry Sidney in 1566, who declared it ‘the greatest of any that we have seen in an Irishman’s possession’. The strategic importance of Sligo Castle intensified during the Tudor conquest; a 1587 map depicts a formidable structure with four towers defending the river crossing.

The castle’s final chapter was marked by violence and eventual obliteration. During the Nine Years’ War, control shifted dramatically between crown forces and Gaelic rebels. In 1595, Red Hugh O’Donnell took the castle but demolished it completely, ensuring ‘he did not leave a stone of it on a stone’ rather than risk it falling back into English hands. Though attempts at reconstruction followed, the castle remained largely ruinous into the early 1600s. By the 1650s, the site had been transformed into a quadrilateral military installation known as the Stone Fort, built to garrison Cromwellian soldiers; a practical end for what had once been Connacht’s most contested stronghold.