The Black Castle ruin stands on a rocky headland over the sea. There is a fine vantage point for views over the town and the coast of North Wicklow from the ruins. The Vikings were extremely active in the Wicklow Town area from around 795 and in 834 they plundered the Monastery at Glendalough.
In the same year they fortified a strategic rocky promonatory at the mouth of the Vartry River in Wicklow town. A castle was subsequently built now known as the Black Castle. There is no mention of the Castle in recorded history until 1174, following the Norman invasion. Between 1295 and 1315 the Castle was attacked and burnt down twice by the local O' Byrne Clan.