The Castletown River is a river which flows through the town of Dundalk, County Louth, Ireland. It rises near Newtownhamilton, County Armagh, Northern Ireland, and is known as the Creggan River in its upper reaches. Its two main tributaries are the and rivers and it enters the Irish Sea at Dundalk Bay on the east coast of Ireland. The Castletown River flows in at the northwest corner of Dundalk Bay creating a deep channel through the intertidal zone. It is routinely dredged for navigation purposes for the deep water port of Dundalk.