What language is this and what does it mean.

It's supposed to be Irish, as you'd probably guessed, although it in fact sounds peculiar in Irish. There are various theories:

Éirinn go brách - Ireland forever, but the grammar in the Irish is so bad that this meaning is doubtful.
Éirinn go breá - Ireland beautiful - but this grammar is even worse.

These are the conventional explanations I've heard, but neither of them is really satisfactory. I think myself it may just have been a phrase which was coined to sound patriotically Irish when being shouted at some volume, but which doesn't really mean anything.

Anyone else?

I see Ó Dónaill has Éire go brách! which means Up Ireland.
kay.

Bryan,
I concur with everyone else, however here are a few tidbits. The most common spelling (or misspelling) is ERIN GO BRAGH, and its most common intended meaning is IRELAND FOREVER. The same slogan has a history of flying on the Green Flag on several Irish Rebellions. The earliest of which I know is 1798. A green flag emblazoned with a harp and this slogan flew over French ships sailing into Mayo, and with some units rising in Wexford. Though I doubt Wallace said this at Stirling Bridge; In the movie BRAVEHEART Mel Gibson shouted ALBA GO BRAGH (To mean Scotland Forever).
Confused yet? (ha ha) One could go on all night with the history of this phrase in the U.S., and Ireland.

Riobárd

Some scots I met use "Albainn go brách" as a toast. I think the "bragh" spelling is based on either the sound in English or an older spelling.
An since it is an exclamation it is hardly suprising that it doesn't fit grammatical rules.
Up Down!
Beir bua
Aonghus

PS Dick Gaughan sings a song about a Scot mistaken for an Irishman in Scotland and nicknamed "Erin go bragh"

Dominic Behan also wrote a song called ERIN GO BRAGH; a rebel song about the Rising in Dublin in 1916.
RIOBÁRD