Granuaile's Castle, Tower house on Clare Island, County Mayo, Ireland.

Granuaile's Castle is a tower house on the east coast of Clare Island, County Mayo, standing 39 feet high. Its two bartizans, corner turrets, were fitted with purple slate flashing when the building was converted into a police barracks in the 1820s. The main living room was on the first floor, with access to the bartizans and the garderobe.

The Ó Máille (O'Malley), Kings of Umaill, built the tower house in the 16th century. It was a stronghold of Gráinne Ní Mháille (Grace O'Malley, c. 1530 – c. 1603), the "pirate queen," who also held Rockfleet and Carrickkildavnet castles. In the 1820s the castle was converted into a police barracks, when purple slate flashing was added to the two bartizans.

The castle takes its name from Gráinne Ní Mháille (Grace O'Malley, c. 1530 – c. 1603), who led the Ó Máille clan and commanded a pirate fleet. It was one of her three strongholds, with Rockfleet Castle on Clew Bay and Carrickkildavnet Castle. The Irish name is Caisleán Ghráinne, "Gráinne's Castle".

The castle is reached by ferry from Roonagh Pier to Clare Island, with the crossing taking around 15 minutes. Spring and summer offer better conditions for visiting, as weather is more stable and more services are available on the island.

The purple slate flashing on the two bartizans dates from the 1820s, when the tower house was turned into a police barracks, not from its 16th-century O'Malley construction. Look for it at the corner turrets, where the main first-floor living room gave access to the bartizans and the garderobe.