Eire is the Irish Gaelic name for "Ireland". Like its English counterpart, the term Éire is used for both the island of Ireland and the Republic of Ireland, the sovereign state that governs 85% of the island's landmass. 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. While Éire is simply the name for the island of Ireland in the Irish language and is sometimes used in English, Erin is a common poetic name for Ireland, as in Erin go bragh.

Source: Wikipedia