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]

Islands of Mayo by population

Demographics

The 2016 census records 14 offshore islands as having a usually resident population. Achill has been connected to the mainland via bridge since 1887, making Clare Island the county's most populous unconnected island, with 159 inhabitants as of 2016. This makes Clare Island the fifth-most populous unconnected island in Ireland, after the three Aran Islands off Galway and Arranmore off County Donegal.^[11]

Mayo County Council is responsible for the governance of the islands, and islanders are entitled to certain benefits and exemptions, such as lower motor tax rates.^[12] The islands also receive funding from the Department of Culture, Heritage and the Gaeltacht, as several offshore islands along with much of northwest Mayo are recognised Gaeltacht areas.

As many as 47 offshore islands had permanent populations prior to the Great Famine. Communities had resided on these islands for generations, and their remains can still be seen to this day through the villages, stone walls and lazybeds which they left behind. By the end of the 19th century most of the county's islands had been deserted. In contrast, Achill benefitted from a bridge to the mainland which allowed the population to temporarily rebound, reaching its all-time peak in 1911.

Emigration from the islands continued into the 20th-century as traditional industries such as fishing and textiles were unable to provide an attractive, or in many cases adequate, income to young islanders. In recent times, the islands have become a popular tourist destination and depend heavily on the tourism and hospitality sector. Their population swells during the summer months, particularly on Achill and Clare. Popular activities include surfing, sailing, island-hopping in Clew Bay, spiritual retreats and eco-tourism.^[13]

The following table shows population trends for the fourteen currently inhabited islands as of the last census. The overall trend is a continuing decline in the population of Mayo's offshore islands, which is currently at a historic low. The county as a whole was one of just three in Ireland to record a population decrease between 2011 and 2016. The population of Mayo during this time decreased by 0.2%, whereas the population of the islands fell by 5.3%. Although the overall population fell during this time, four previously uninhabited islands received new residents, including Achillbeg, which had been uninhabited since 1965.^[14]

Currently inhabited islands

Formerly inhabited islands

Islands of Mayo by area

Larger islands

With the exception of Achill, County Mayo's islands are generally small in size, with only 13 larger than 100 acres (40 ha) and 3 larger than 1,000 acres (400 ha). There are numerous offshore islands in the 10 to 100 acre range, along with at least 15 freshwater islands larger than 10 acres, the largest of which is Illannaglashy on Lough Conn at 82 acres (33 ha).^[15]

The one hundred largest islands in the county are listed below. "F" designates a freshwater island.

Smaller offshore islands

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.

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.

Lough Mask

• Note: The vast majority of Lough Mask is within County Mayo, with a small southern portion of the lough located in County Galway. This list only includes islands which are within the boundary of County Mayo.

Lough Carra

Lough Corrib

• Note: The vast majority of Lough Corrib is within County Galway, with a small northern portion of the lough located in County Mayo. This list only includes islands which are within the boundary of County Mayo.

Lough Conn

Smaller lakes

Lough Cullin

• Griffins Island
• Illanboy
• Illaner
• Illangub
• Illanneill
• Illanulque

Lough Beltra

• Bush Island
• Coarse Island
• Islandmore
• Low Island
• Tree Island

Carrowmore Lake

• Atlavally Island
• Derreens Island
• Gortmore Island
• Muingerroon Island

Furnace Lough

• Illanroe
• Inishower
• Saints Island

Islandeady Lough

• Note: Despite being a small lough, Islandeady had three sizeable islands, at 38, 21 and 20 acres respectively. However, due to the lowering of waterlevels in the 20th century the three islands are now peninsulas.
• Illanlteige East
• Illanteige West
• Islandeady

Monastic islands

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.

Biodiversity

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.

Special areas of conservation

• Mullet/Blacksod Bay Complex
• West Connacht Coast
• Erris Head
• Inishkea Islands
• Duvillaun Islands
• Broadhaven Bay
• Killala Bay/Moy Estuary
• Croaghaun/Slievemore
• Achill Head
• Keel Machair/Menaun Cliffs
• Doogort Machair/Lough Doo
• Clew Bay Complex
• Corraun Plateau
• Lough Gall Bog
• Clare Island Cliffs

Special protection areas

Italics indicates that the SPA is an inland lake

• Blacksod Bay/Broadhaven Bay
• Inishglora and Inishkeeragh
• Inishkea Islands
• Duvillaun Islands
• Stags of Broad Haven
• Illanmaster
• Killala Bay/Moy Estuary
• Doogort Machair
• Bills Rocks
• Clare Island
• Mullet Peninsula
• Termoncarragh Lake & Annagh Machair
• Lough Mask
• Lough Conn & Lough Cullin
• Lough Carra
• Carrowmore Lake

Natural heritage areas

• Tullaghan Bay & Bog
• Doogort East Bog
• Sraheens Bog

Proposed natural heritage areas

• Killala Bay/Moy Estuary
• Glenamoy Bog Complex
• Stags Of Broadhaven
• Broadhaven Bay
• Erris Head
• Eagle Island
• Mullet/Blacksod Bay Complex
• Inishglora & Inishkeeragh
• Inishkea Islands
• Duvillaun Islands
• Croaghaun/Slievemore
• Doogort Machair/Lough Doo
• Inishgalloon
• Keel Machair/Menaun Cliffs
• Corraun Plateau
• Lough Gall Bog
• Bills Rocks
• Clare Island
• Clew Bay Complex
• Mweelaun Island
• Caher Island
• Ballybeg Island
• Inishturk
• Inishdalla
• Frehill Island
• Inishdegil Islands

Island index

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