Erin is a Hiberno-English word for Ireland originating from the Irish word "Éirinn". "Éirinn" is the dative case of the Irish word for Ireland, "Éire", genitive "Éireann", the dative being used in prepositional phrases such as "go hÉirinn" "to Ireland", "in Éirinn" "in Ireland", "ó Éirinn" "from Ireland".

The dative has replaced the nominative in a few regional Irish dialects (particularly Galway-Connemara and Waterford). Poets and nineteenth-century Irish nationalists used Erin in English as a romantic name for Ireland. Often, "Erin's Isle" was used. In this context, along with Hibernia, Erin is the name given to the female personification of Ireland, but the name was rarely used as a given name, probably because no saints, queens, or literary figures were ever called Erin.

According to Irish mythology and folklore, the name was originally given to the island by the Milesians after the goddess Ériu.

The phrase Erin go bragh ("Éire go brách" in standard orthography, dative "in Éirinn go brách" "in Ireland forever"), a slogan associated with the United Irishmen Rebellion of 1798, is often translated as "Ireland forever".