Answer:
It doesn't mean either of those.
Erin is an anglicsation of "Éirinn". Éirinn means "Ireland" (it's the dative of Éire).
go brágh should be "bráth / brách" (brágh is Gàidhlig).
"Go bráth" means "until Judgement day"
Bráth comes from the old Irish word "to judge" (the modern word is breith).