When did the term 'Erin Go Braugh' originate?

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