Erin is a Hiberno-English derivative of the Irish word "Éirinn". ("Éirinn" is the dative case of the Irish word for Ireland - "Éire", genitive "Éireann").
Poets and nineteenth-century Irish nationalists used Erin in English as a romantic name for Ireland. In this context, along with Hibernia, Erin is the name given to the female personification of Ireland.
According to Irish mythology and folklore, the name was originally given to the island by the Milesians after the goddess Ériu.
Erin go bragh ("Éirinn go brách" in standard orthography), a slogan dating from the 1798 revolution, is often translated as "Ireland forever". The etymology of the word as it drifted throughout the Gaelic region gave rise to its use by the early Scots to both mean Ireland and "west" - as Ireland lies to the west of Scotland.
Read more about Erin: Usage As A Given or Family Name