At full tide, and with the right weather, the lagoons between the Bull Island and the coast are indeed blue. Shortly after the island was declared Ireland’s first bird sanctuary in 1931, a plan was hatched to maintain a Blue Lagoon at all tide levels and develop the island for tourism, though it never took off.

At low tide, one reason for the island’s 1981 designation as a UNESCO Biosphere Reserve, extended in 2015 to all of Dublin Bay, becomes clear. As mud.