Construction began in 1269 under Crown authority, but the O’Conor clan had other ideas; they attacked and burned it three times before the builders finally completed it around 1290. The castle’s strategic importance meant it rarely knew peace, changing hands repeatedly between various O’Conor factions, Crown forces, and local lords throughout its active life. After suffering damage from Felimid O’Conor in 1305, capture by Ruarai O’Conor in 1315, and another seizure by Turlough O’Conor in 1540, the fortress became a political bargaining chip. The O’Conors controlled it from about 1360 to 1566, though Richard II pragmatically appointed O’Conor Don as constable in 1394, essentially acknowledging who really held the power.

The castle’s remains reveal sophisticated medieval military architecture, consisting of a rectangular inner ward measuring 53 by 38 metres internally, defended by D-shaped towers at each corner. These towers originally rose three storeys high, except for the northwest tower which had only two. The surviving southeast and southwest towers still display their original thirteenth-century features, including arrow loops, fireplaces, and garderobes; practical necessities for both defence and daily life. The eastern gatehouse, formed by two D-shaped towers creating a formidable entrance passage, and the western postern gate with its murder hole above the vaulted passage, demonstrate the builders’ attention to defensive detail. Archaeological investigations have revealed that the castle originally had two wards, with the outer ward defined by water-filled fosses (defensive ditches) and masonry walls, creating multiple lines of defence against attackers.

The castle’s later history reflects changing military needs and social aspirations. Sir Nicholas Malby, granted the castle in 1577, or possibly his son Henry, transformed portions of it into a more comfortable fortified house by inserting large mullioned windows with decorative hood mouldings into the gatehouse and northern towers. Despite these improvements, conflict continued to find Roscommon Castle; it was burned by O’Donnell in 1596 and 1599, garrisoned by Parliamentarians in the 1640s, surrendered to Confederate forces after a battle in 1645, and handed over to Cromwell’s forces without a fight in 1652. A final fire in 1691 ended its active use, leaving the impressive ruins that now stand under state guardianship as National Monument No. 181, positioned dramatically on the edge of a turlough (seasonal lake) about 800 metres north of St Coman’s church.