() is Irish for "Ireland", the name of both an island in the North Atlantic and the sovereign state of the Republic of Ireland which governs 84% of the island's landmass. The latter is distinct from Northern Ireland, which covers the remainder of the northeast of the island. The same name is also sometimes used in English ( ).
Etymology
The modern Irish Éire evolved from the Old Irish word Ériu, which was the name of Ireland and of a Gaelic goddess. Ériu is generally believed to have been the matron goddess of Ireland, a goddess of sovereignty, or simply a goddess of the land. Ériu has been derived from reconstructed Archaic Irish *Īweriū, and further from the Proto-Celtic *Φīwerjon- (nominative singular Φīwerjū). This suggests descent from the Proto-Indo-European *piHwerjon- ("fertile land" or "abundant land"), from the adjective *piHwer- ("fat") – cognate with Ancient Greek píeira and Sanskrit pīvarī, ("fat, full, abounding"). The Archaic Irish form was borrowed into Ancient Greek. During his exploration of northwest Europe (circa 320 BCE), Pytheas of Massilia called the island Ierne (written ). The Pseudo-Aristotelian text On the Universe (393b) has: Translation: There are two very large islands in it, called the British Isles, Albion and Ierne. (modern Great Britain and Ireland). In his book Geographia (circa 150 CE), Claudius Ptolemaeus called the island Iouernia (written ; ou represented /w/) and named a tribal group called the (, or Iverni who lived in the southwest. This was borrowed into Latin as Hibernia.
The evolution of the word would follow as such: Proto-Celtic *Φīwerjon- (nominative singular *Φīwerjū) Archaic Irish *Īweriū Old Irish Ériu Modern Irish Éire
An Old Irish by-form of this placename was íriu, meaning "land, earth". In Irish mythology, Íth is the first of the Milesians (Irish) to see Ireland from Iberia. Later, he is the first to step ashore and praises the island's abundance, saying to the Tuatha Dé Danann: "You dwell in a good land. Abundant are its mast and honey and wheat and fish".
A 19th-century proposal, which does not follow modern standards of etymology, derives the name from Scottish Gaelic: ì (island) + thiar (west) + fónn (land), which together give ì-iar-fhónn, or "westland isle". The etymology fails in that tiar (the historical form) has no *téir forms which would allow the development of the *é of Éire; moreover, Old Irish í ("island") was a late loanword from Old Norse ey ("island"), and so did not exist in prehistoric Ireland.
Difference between Éire and Erin
While Éire is simply the name for the island of Ireland in the Irish language (gaeilge), and sometimes used in English, Erin is a common poetic name for Ireland, as in Erin go bragh. The distinction between the two is one of the difference between cases of nouns in Irish. Éire is the nominative case, the case that is used for nouns that are the subject of a sentence, i.e., the noun that is doing something as well as the direct object of a sentence. Erin derives from Éirinn, the Irish dative case of Éire, which has replaced the nominative case in Déise Irish and some non-standard sub-dialects elsewhere, in Scottish Gaelic (where the usual word for Ireland is ) and Manx (like Irish and Scottish Gaelic, a Goidelic Celtic language), where the word is spelled "Nerin," with the initial n- probably representing a fossilisation of the preposition in/an "in" (cf. Irish in Éirinn, Scottish an Èirinn/ann an Èirinn "in Ireland"). The genitive case, Éireann (e.g. stair na hÉireann "the history of Ireland, Ireland's history"), is found in the Gaelic forms of the titles of companies and institutions in Ireland e.g. Iarnród Éireann (Irish Rail), Dáil Éireann (Irish Parliament), Poblacht na hÉireann (The Republic of Ireland) or Tuaisceart Éireann (Northern Ireland).
As a state name
Article 4 of the Irish constitution adopted in 1937 by the government under Éamon de Valera states "Éire is the name of the state, or in the English language, Ireland". The Constitution's English-language preamble also describes the population as "We, the people of Éire". Despite the fact that Article 8 designated Irish as the "national" and "first official" language, Éire has to some extent passed out of everyday conversation and literature, and the state is referred to as Ireland or its equivalent in all other languages. The name "Éire" has been used on Irish postage stamps since 1922; on all Irish coinage (including Irish euro coins); and together with "Ireland" on passports and other official state documents issued since 1937. "Éire" is used on the seal of the president of Ireland.