Are they just different translations of Irish?
Erin is an Anglicization of Eireinn, which shows the obvious connection to “Eire.” Both literally mean “Ireland” but Erin is more often applied to the island as a landform and “nation” in the ethnological sense, and Eire to the Republic as political entity. I have been told by people I generally trust that they derive from the Old Irish for “peace,” cognate with Greek iréné, but offer this only as passing on unconfirmed information.
Éire is the Irish word for Ireland. It is also used by non-Irish people to refer to the territory of the Republic of Ireland (i.e. by contrast with the North). This tends to annoy Irish people, for reasons that are hard to explain. It is not usually used by Irish people when speaking English.
Erin is hardly used at all in Ireland in my experience (except in brand names and the name of a GAA club). Certainly not as a way of referring to the country. It is familiar to us from emigrant songs and slogans like “Erin go bragh”. It is similar to the Irish “Éireann” which is a genitive and illative form of “Éire”.
I’ve always thought Erin was just the poetic version of Eire. Perhaps that comes from having sung *Hail Glorious Saint Patrick * too many times.
Sorry, scratch that “illative” - I didn’t mean to imply there is a well-developed system of noun case endings in Irish. What I am trying to describe is irregular forms like “in Éirinn” that are used when you use prepositions like “in” and “to” Ireland.
I think that’s exactly right, Cunctator.
The mythical origin of the name is that there were three queens of the Tuatha Dé Danann, Ériu, Banba and Fódla. The name of the country was to alternate on a three-year cycle, but ended up sticking with Éire.
If you were looking for an Indo-European root, I think “peace” may be a bit optimistic. It could be cognate with the word “Arya” (as in “Aryan”), which is related to the word “art”.
Is Wiki wrong on this? They say the Entomology of Erin is rooted in the second wave of Celtic speakers circa 500 B.C.
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These were the Builg or Érainn. The former name (originally Bolgi) identifies them as Belgae*, a Celtic people mentioned by Julius Caesar. Their other name (originally Iverni) is probably the origin of several of the early Classical names for Ireland: the Greek Ιερνη; Ιουερνια and possibly also the Latin Hibernia. In Irish mythology the name Fir Bolg obviously refers to the same people.
*A people who formerly inhabited northeast Gaul and areas of southeast England. Belgium is named for them.
Yes and no. Whoever wrote the Wikiarticle was basing it on equating scanty historical and anthropological detail with the basis behind Irish legend. The Firbolg are legendary. That they may have been identical to the Belgae or the Iverni is very reasonable speculation, but speculation nonetheless.
(BTW, I completely defer to Hibernicus as regards the content of post #2. My “Eireinn” was a misspelling/misremembering for Eireann, and most of the content of my post addressed what I had heard the terms generally used as, here in America. Obviously his post on what the Irish people actually do should supersede that.)
To me, they mean exactly the same thing: A 4 letter word pertaining to anything Irish. But then again, I do way too many crossword puzzles. 
Where I come from, Erin is a girl’s name, and we don’t know how to pronounce Éire.
How is Éire pronounced, anyways? “Air”?
AIR-ah.
As hibernicus pointed out, Eireann is the genitive form of the noun, eg “muintir na hEireann”, people of ireland. And I’ll also agree that Erin isn’t used in Ireland…
…who brought the potato bugs with them.
Dang it Beware of Doug! Of course I meant “Etymology” … once that post went up like that I sweated and I sweated – nothing last night and so I thought it was going to die — I know everyone else was too polite to laugh and point – but at least i could pretend that no one noticed 
Anyway … does anyone dispute that the word that is in English approximately “Erin” began to refer to Ireland to some European folks about 500 BC and is probably rooted in the name of a single Celtic tribe?
The first coupla weeks in March always bring references to the Erin Go Bra® brand name. Two years ago, that trade mark was bought by Playtex.