Castleknock Castle, Norman castle ruins in Fingal, Ireland.

Castleknock Castle is a ruined Norman castle in Castleknock, Dublin, now on the grounds of St. Vincent's Castleknock College. Its polygonal keep stood about 24 m high, attached to a large squat building, and was surrounded by a curtain wall with towers and two deep moats. The enclosed plot within the old walls is now a cemetery.

Hugh Tyrrell, the first Baron of Castleknock, built this fortress in the 12th century to control western approaches to Dublin. The structure served as a key defensive position throughout the medieval period.

The Irish name Caisleán Cnucha comes from Cnucha, the form used in the Annals of the Four Masters, the Annals of Ulster and the Annals of Tigernach. A legend tells of Eibhleen O'Brinn, daughter of a Wicklow chieftain, imprisoned in the turret by the Baron's brother; a white-robed figure was said to walk the tower at midnight, called the White Lady of Castleknock.

The ruins are located within a private boys' school grounds and are most visible during winter months when vegetation is sparser. Plan your visit for the colder season to see the structures more clearly as summer growth obscures the details.

In 1861 an ancient altar tomb known as a cromlech was found inside the castle walls, evidence the hill was used before the Normans arrived. The enclosed ground within the old walls now serves as a cemetery. The ruins sit among trees and are hidden from the road except in winter.