Mutton Island is located about 800 metres offshore from Galway City. About half way between Nimmo Pier and Salthill, there is a causeway that links the island to the shore. It is a popular walk in Galway. At low tide, the shoreline is exposed on both sides of the causeway and this attracts a wide variety of birds.
There is no access to the island at the end of the causeway, but there is plenty to see from the causeway itself. If you have parked in the city centre to check out Nimmo Pier, you can easily get to the start of the causeway from the end of Nimmo Pier. Just follow the path along the shoreline. Otherwise you can find parking on the road leading to Salthill and walk back past the Famine Ship Memorial to the causeway.
This is a top location to go birdwatching in Galway.
Parking at Salthill: 53.262718, -9.070252
If you have the time you should also visit Lough Atalia