Newcastle takes its name from the Castle, constructed in the period between 1177 and 1184 by Hugh de Lacy then governor of Ireland under Henry II. The castle became a major stronghold in the outer fortifications of the Pale. Even so it was attacked and occupied at intervals by the Wicklow Irish chieftains the O’Tooles and the O’Byrnes.
The current ruin on the site is not that of the castle which was destroyed in the 1500’s but of a building which was erected on the site. The Castle was the county seat and administrative centre until the shiring of Wicklow county in the 1500’s when it moved to Wicklow Town after the castle was again raided.