This articles lists the islands of County Mayo, the mainland of which is part of the island of Ireland. Included in this list are named offshore and freshwater islands as recorded by Ordnance Survey Ireland or the Placenames Database of Ireland. Additionally, areas of ecological significance related to both offshore and freshwater islands, designated by the National Parks and Wildlife Service, are also listed.

Although County Mayo has hundreds of offshore and freshwater islands, only a handful of islands and island groups are large enough to be distinguishable on a typical map of the county, namely Achill Island, Clare Island and Inishturk, along with the island clusters of Duvillaun, Inishkea, Clew Bay and the major loughs. There are over 100 islands which are greater than 10 acres (4.0 ha) in area. The largest island is Achill, which extends to 148 km² (37,000 acres), making it Ireland's largest offshore island.^[2]

Due to its abundance of bays, inlets and offshore islands, Mayo is reputed to have the longest coastline of any county in Ireland, at 1,168 km (726 mi) or approximately 21% of the total coastline of the State.^[3][4]

The geology and geomorphology of the county and its islands is among the most varied and complex in Ireland. Islands such as those at Clew Bay are low-lying, formed when rising sea levels drowned a vast field of drumlins, while others like Clare Island and Achill are characterised by mountains and towering sea-cliffs, of which Croaghaun are the third-highest in Europe, at 688 m (2,257 ft)^[5]

The bedrock geology of the islands is broadly divided into two groups. Those north of Clew Bay such as Achill and the islands off the Mullet Peninsula are generally of ancient Dalradian age, similar to County Donegal and the Grampian Highlands. Inishglora contains some of the oldest rocks in Ireland, at 1.75 billion years old. Further south, the islands are much more diverse. Clare island alone is composed of Cambrian, Ordovician, Silurian and Carboniferous terranes sutured together. Geologically, the Clew Bay islands themselves are the youngest in the county, formed during the Lower Carboniferous about 350 million years ago.^[6]

The county's offshore islands - and islands off the west coast of Ireland generally - developed their own distinct culture and traditions stretching back centuries. Records from the 1841 Census show that 47 of County Mayo's offshore islands were inhabited, with a combined population of approximately 10,000.^{[n 1][7]} The Great Famine resulted in widespread or total depopulation of the islands. Towards the end of the 19th century the population of the larger islands began to recover somewhat, only to be set back by emigration following independence as the Irish government initially encouraged the evacuation of the islands due to the cost of providing services to them, further hastening the decline.^[8]

Today, the islands are recognised by the State as an integral part of Ireland's national heritage, preserving the remnants of a unique aspect of Irish culture. The remaining inhabited islands receive support and funding through the Department of Culture, Heritage and the Gaeltacht.^[9] As of the 2016 Census, there are 2,700 people living on 14 of the county's offshore islands (of which 2,440 live on Achill), accounting for 1.87% of the county's population.^[10]

County Mayo has hundreds of smaller islands and islets. Most of the county's offshore islands are located in Clew Bay, with smaller clusters located around the Achill archipelago, the west and south coasts of the Mullet Peninsula, and Blacksod Bay. The list below is not exhaustive, and includes named offshore islands that are either extensive (typically greater than 2 acres in area) or in someway locally significant or identifiable.

Unlike the above list, this list is in alphabetical order and does not include freshwater islands.

There are numerous large loughs within the county, containing hundreds of small freshwater islands. Lough Mask in southern Mayo is the largest lough in the county. At 83 km² (21,000 acres) it is the 6th-largest lough in Ireland (as well as the 6th-largest in Britain and Ireland). Further south, Lough Corrib is the 2nd-largest lough in Ireland; however, only a small portion of this lough is located within the county.^[16]

Illannaglashy, on Lough Conn, is the largest freshwater island in the county, at 82 acres (33 ha) in area. Freshwater islands have played an important role in the county's history and host an abundance of castles, monasteries, church ruins and unspoiled woodlands.

Six islands within County Mayo have had a dedicated monastery or abbey established on them. Four of these are offshore, and two of these are freshwater islands. They are listed below:

• Church Island Monastery (F)
• Clare Island Abbey
• Duvillaun Monastery
• Inishkea North Monastery
• Inishglora Monastery
• Partry Monastery (F)

In addition to the above sites, many more islands within the county have been used for religious purposes. The ruins of old churches and cemeteries can be found scattered across such islands.

Many of the county's offshore and freshwater islands are designated as areas of ecological significance and are protected both at national and European level. Mayo's offshore islands are a haven for wildlife and biodiversity, supporting hundreds of species of bird, fish and marine mammal, as well as unique vegetation. Five of the county's major inland lakes (along with their islands) are also protected, those being Lough Mask, Lough Conn, Lough Cullin, Lough Carra and Carrowmore Lake.^[17]

The biodiversity of the islands is protected under the following designations. There is overlap across designations, as different designations apply to different species, e.g. the Duvillaun Islands are an SAC, SPA and proposed NHA.

• Special Area of Conservation (SAC) - These are prime wildlife conservation areas in the country, considered to be important on a European as well as Irish level.
• Special Protection Area (SPA) - Classified under the EU Birds Directive for the protection of endangered species of wild birds.
• Natural Heritage Area (NHA) - This is an area considered important for the habitats present or which holds species of plants and animals whose habitat needs protection.

The following areas of ecological significance are located either on or adjacent to County Mayo's offshore islands. The five major inland lakes listed as SPAs are also included, and are shown in italics in the list below.

• Achill Island
• Clare Island
• Inishturk
• Inishkea North
• Inishkea South
• Achillbeg
• Annagh Island
• Caher Island
• Barranagh Island
• Inishbiggle
• Eagle Island
• Inishglora