Carlow Castle, 13th-century castle ruin near River Barrow in Carlow, Ireland.

Carlow Castle is a ruined towered keep near the River Barrow in County Carlow, Ireland. It was built between 1207 and 1213, and only the west wall and two neighbouring towers remain. The keep originally had four three-quarter-circular corner towers, stood three storeys high, and was built of limestone.

William Marshal, Earl of Pembroke, built the castle in the early 13th century to control a key crossing on the River Barrow during the Norman settlement of Ireland. Over the centuries it served as a seat of administration and held its role as a stronghold until it gradually fell out of use.

The castle sits right in the town center of Carlow, making it a daily landmark that locals pass by without a second thought. Visitors can walk close to the remaining towers and get a direct look at how Norman builders worked with stone.

The ruin is in the town center of Carlow and easy to reach on foot from most points in town. The remaining structures are compact, so a visit does not take long and fits naturally into a walk around the area.

In 1814 a local doctor tried to convert the castle into a psychiatric hospital, and an explosion during the work brought down much of what was still standing. That accident is the reason the ruin has the irregular shape it does today.