It is bounded on the north and west by the Atlantic Ocean; to the south by County Galway; the east by County Roscommon; and the northeast by County Sligo. Mayo is the third-largest of Ireland's 32 counties in area and 15th largest in terms of population. It is the second-largest of Connacht’s five counties in both size and population. Mayo is located along the west coast of Ireland which is approximately 7,400 km in length. There is a distinct geological difference between the west and the east of the county. The west consists largely of poor subsoils and is covered with large areas of extensive Atlantic blanket bog, whereas the east is largely a limestone landscape. Agricultural land is therefore more productive in the east than in the west.
- The highest point in Mayo (and Connacht) is Mweelrea, at 814 m (2,671 ft)
- The River Moy in the northeast of the county is renowned for its salmon fishing
- Ireland's largest island, Achill Island, lies off Mayo's west coast
- Mayo has Ireland's highest cliffs at Croaghaun, Achill Island, while the Benwee Head cliffs in Kilcommon Erris drop almost perpendicularly 900 feet (270 m) into the Atlantic Ocean.
- The northwest areas of County Mayo have some of the best renewable energy resources in Europe, if not the world, in terms of wind resources, ocean wave, tidal and hydroelectric resources
Local government and political subdivisions
Mayo County Council (Irish: Comhairle Contae Mhaigh Eo) is the authority responsible for local government. As a county council, it is governed by the Local Government Act 2001. The County is divided into four municipal areas Castlebar, Ballina, Claremorris and West (an area which stretches from Westport to Belmullet). The council is responsible for housing and community, roads and transportation, urban planning and development, amenity and culture, and environment. For the purpose of local elections the county is divided into its four municipal districts, replacing the former six local electoral areas: Ballina (6), Belmullet (4), Castlebar (7), Claremorris (6), Swinford (4) and Westport (4). The county town is at Áras an Contae in Castlebar, the main population centre located in the centre of the county. For national elections, half of the Claremorris Municipal District is in Galway West, and stretches from Ashford Castle to Ireland West Airport.
The Municipal Area populations are as follows:
- Ballina – 32,979
- Castlebar – 34,000
- Claremorris – 32,469
- West Mayo – 31,190
Near Roonagh Quay looking towards Croagh Patrick
There are nine historic baronies, four in the northern area and five in the south of the county:
- Erris (north-west, containing Belmullet, Gweesalia, Bangor Erris, Kilcommon, Ballycroy etc.)
- Burrishoole (west, containing Achill, Mulranny and Newport, County Mayo)
- Gallen (east, containing Bonniconlon, Foxford)
- Tyrawley (north-east, containing Ballina, Ballycastle, Killala, Moygownagh)
- Clanmorris, (south-east, containing Claremorris and Balla)
- Costello (east-south-east, containing Kilkelly Ballyhaunis) etc.
- Murrisk (south-west, containing Westport, Louisburgh, Croagh Patrick etc.)
- Kilmaine (south, containing Ballinrobe, Cong etc.)
- Carra (south, containing Castlebar, Partry etc.)
Largest towns by population (according to the 2016 Census)
- Castlebar – 13,496
- Ballina – 10,623
- Westport – 5,894
- Claremorris – 4,487
- Ballinrobe – 3,685
Flora and fauna
A survey of the terrestrial and freshwater algae of Clare Island was made between 1990 and 2005 and published in 2007. A record of Gunnera tinctoria is also noted.
Consultants working for the Corrib gas project have carried out extensive surveys of wildlife flora and fauna in Kilcommon Parish, Erris between 2002 and 2009. This information is published in the Corrib Gas Proposal Environmental impact statements 2009 and 2010.