O'Brien's Castle, Medieval tower house in Inisheer, Ireland.

O'Brien's Castle, also called Furmina Castle, is a rectangular stone keep on a high point in the north of Inisheer, County Galway, Ireland. Its original doorway was on the first floor, reached by an external wooden stair. The first floor formed a great hall with a mural stair to the parapet, while the ground floor held three vaulted storage chambers. A corbel of a bartizan carved with a human face is visible.

The O'Brien family built the tower around 1400 on an island that had already been settled for centuries. The O'Flaherty clan took control in 1582, and Cromwell's forces damaged the structure in 1652, ending its use as a military site.

A carved human face on a corbel stone inside the tower shows how medieval builders added personal touches even to defensive structures. This kind of detail is rare and gives visitors a direct, tangible connection to the craftsmen who worked here.

The tower sits on a rocky rise inland from the ferry pier on Inisheer and can be reached on foot, though the path goes uphill. Inside, the ground is uneven and the stairs are steep, so sturdy footwear is a good idea before heading up.

The tower was built directly on top of an ancient ringfort called Dún Formna, which was in use from around the 5th century BC to the 1st century AD. This means people considered this exact spot worth defending for well over a thousand years before the first stone of the tower was laid.