Erris is a barony in northwestern County Mayo in Ireland consisting of over 230,452 acres (932.61 km²), much of which is mountainous blanket bog. It has extensive sea coasts along its west and north boundaries. The main towns are Belmullet and Bangor Erris. The name Erris derives from the Irish 'Iar Ros' meaning 'western promontory'. The full name is the Iorrais Domnann, after the Fir Bolg tribe, the Fir Domnann. To its north is the wild Atlantic Ocean and the bays of Broadhaven and Sruth Fada Conn and to its west is Blacksod Bay. Its main promontories are the Doohoma Peninsula, Mullet Peninsula, Erris Head, the Dún Chiortáin and Dún Chaocháin peninsulas and Benwee Head.

There are five Catholic parishes in Erris: Kilcommon, Kilmore, Kiltane, Belmullet and Ballycroy.

Parts of Erris are in a Gaeltacht area, with first-language speakers of Irish in the following areas of the barony: An Fál Mór, Tamhaiin na hUltaí, Glais, Eachléim, Tearmann, Tránn, An Mullach Rua, Cartúr, An Baile Úr, Cill Ghallagáin, An Corrán Buí, Ceathrú na gCloch, Port a' Chluaidh, Ros Dumhach and Ceathrú Thaidhg. The area with the most Irish speakers is Ceathrú Thaidhg. There are between 3,500-5,000 native Irish speakers in Erris.^{[citation needed]}

Much of inland Erris is covered with blanket bog. A triangle between Ballycroy, Bellacorick and Bangor Erris consists of little else over its surface. Blanket bog, unlike raised bog, grows slowly across the landscape like a blanket covering the ground, due to a continuous supply of water from rainfall, maintaining waterlogged conditions on the ground. The bog is acidic with a pH of between 3.5 and 4.2. This is Atlantic blanket bog and it provides a suitable habitat for many species of flora, particularly small species of mosses, bryophytes, carnivorous plants and delicate flowers such as the scarlet pimpernel. Species of fauna found on Atlantic blanket bog, include smaller varieties such as frogs and insects as well as many bird varieties, not common elsewhere.

Several areas of the blanket bog are protected under European legislation such as Special Areas of Conservation, Special Protection Areas (Birds Directive) and Natural Heritage Areas. These include the Glenamoy Bog Complex, Ballycroy National Park and Bellacorick.

Erris has a large range of habitats including blanket bogs, estuaries, salt marshes, fresh water lakes, coasts, cliffs, machair, sand dunes, sandy beaches and rocky shores. It is an important area for bird watchers as the largely treeless landscape allows relatively easy access for birdwatching. Brent geese overwinter here feeding along the estuaries, and corncrake, chough, rock dove and twite are sometimes seen at Erris Head. Sightings of rarer birds, such as the grey phalarope, booted warbler and Wilson's petrel, have also been recorded.

The oldest rocks in Ireland, some 1.8 billion years old, are to be found on the west coasts of Erris. Pink or orange striped gneisses are found along the beaches of Elly Bay and Annagh Head where they have become separated from the same rocks on the east coast of Northern Canada over hundreds of millions of years by the separation of tectonic plates in the mid Atlantic Ocean. "Erris Head" (Irish: Ceann Iorrais) is a promontory at the northernmost tip of the Mullet Peninsula. It is a landmark known well by mariners and is one of the sea areas cited by Met Éireann's weather forecasters. The coastline of Erris has some of "the grandest sea cliffs in Ireland" over the Atlantic Ocean from where the next stop is the east coast of America. Erris Head is not served by any road and can only be reached by crossing a number of fields from the hamlet of Glenlara where the road ends.

Along the coast there are several uninhabited islands. These include the Inishkea Islands, Inishglora, Duvillaun, the Stags of Broadhaven Bay and other smaller islands.