The castle sits on the east shore of Clare Island, on a low rise above the water with a clear view across to the Mayo mainland. It is a few miles around the headland from the modern harbour where the ferry from Roonagh Quay lands, and it commands the approach that any vessel coming up Clew Bay from the south would have used. The siting is purely tactical: from this tower, the O'Malleys could see ships coming from a long way off and were positioned to put their own galleys to sea quickly. The first floor view east on a clear day takes in Croagh Patrick, the drumlins of Clew Bay, and the small islands that legend numbers at one for every day of the year.

Grace O'Malley - born around 1530, dead around 1603 - inherited the O'Malley clan's maritime tradition and built it into something formidable enough that Queen Elizabeth I granted her an audience at Greenwich in 1593. She operated from three tower houses on the Mayo coast: Granuaile's Castle on Clare Island, Carrickkildavnet Castle on Achill, and Rockfleet Castle on Clew Bay (her favourite, where tradition holds she died). All three are recognisable as variations on the same architectural type - the late medieval Irish tower house - but their settings tell three different stories about how she controlled this coast. Clare Island was her bastion: the offshore stronghold, the place she could not easily be reached. Carrickkildavnet controlled the chokepoint at Achill Sound. Rockfleet controlled the inner waters of Clew Bay. Together they let her see, intercept, and respond to anything moving on the coast.

After the O'Malley power faded in the seventeenth century, the castle stood empty for generations until the Royal Irish Constabulary needed a barracks on Clare Island in the 1820s. They moved into Granuaile's Castle and adapted it to their purposes - sealing some openings, adding the purple slate flashing to the bartizans that still marks the building today, and installing the kind of utilitarian fittings a police post required. The arrangement was, in its own way, characteristic of how British rule used existing Irish structures: the same tower that once enforced O'Malley authority over the sea was now enforcing crown authority over O'Malley descendants. The barracks period lasted until the early twentieth century, when the building was again abandoned to weather and stone.

Granuaile's Castle is a National Monument, accessible to visitors, with the main living room on the first floor opening to the two bartizans and a small garderobe (the medieval term for what was effectively the toilet, hung out from the wall over open air). The defensive features that made the tower habitable in the sixteenth century - thick walls, narrow openings, the steep internal stair - are still legible. The view from inside, framed by the small windows the tower's builders considered an acceptable concession to light, gives a strong sense of how the world looked to its occupants: contained, watchful, oriented toward the water. It is not a romantic building. It is a working structure that has outlasted its work by four hundred years.