The Castletown Estuary is a tidal, saltwater feature forming part of Dundalk Bay in County Louth. It is formed by the Castletown River, which meanders through Dundalk Harbour before reaching the sea. The estuary is characterized by extensive intertidal sand and mud flats, with "literally miles of sand are exposed at low tide" across Dundalk Bay. This creates a significant tidal range, dramatically altering the landscape between high and low water. The water body is very shallow, seldom deeper than eighteen metres at low tide even in the wider bay. For paddlers, this means careful tide planning is essential; launching or retrieving a small boat at locations like Blackrock could require waiting up to ten hours for suitable water levels, indicating a long tidal window. The best access points for the wider bay are Blackrock and Annagassan. At Annagassan, a slipway on the southern side allows for small boat launch, though high water presents the best conditions. The estuary's banks feature a loop walk along its lower reaches, indicating developed footpaths. The overall water quality status of the Castletown Estuary is currently classified as poor, with waste water discharges identified as a significant pressure. The area is an important wetland for waterbirds, including gulls, waders, and duck, making it of interest for wildlife observation. The estuary holds historical significance for local surfing, with a reference to a boy paddling a board up it in 1949. A key safety consideration is the speed of the tide, which can come in very fast across the mudflats, potentially cutting off the unwary. The area is also used by charter boats operating from Bellurgan within the estuary, indicating the presence of other vessel traffic.