The first thing you need to understand is the trinity of sisters
known as albana,banba and of course Erin. The original term of this
statement was banba go brea banba being one of the original names
of Ireland. It was used primarily as a battle cry espicealy during
the Norman invasions. The scottish also used there own form of this
cry and it was albana ga braugh. Albana being one of the names for
Scotland. To answer the rest of your question the name Erin being
used as a term for Ireland only started to come about in the 1200's
though by the time it was used in the feinan rebellion it was well
known. In terms of what it actually means that's more complecated
due to the fact that Irish words in genrel have about ten different
meanings and the only way to know what is being said is to
understand the context it's being used in go brea can mean good,
wonderful, excellent, etc.. but in this context it means only what
the sayer is using the words for it's either a rebel cry, a
statement of the love of Ireland itself and can also be used to
take the mick as in Irelands wonderful really. Hope this helps