It's an Irish Republican rebel ballad from the 1840s. The reason I know is because I was once in a bar in Liverpool and a couple of lads started singing it and a couple others objected and a fight broke out. As a loyal subject of the Crown, I was on the side of the objectors. We eventually prevailed, but, even if we hadn't, 'A Nation Once Again' is a fine song to get your head kicked into, at least when compared to 'Believe' by Cher, which would rank pretty high on the list of numbers I'd least like to be listening to as my eye's gouged out and I fall into a coma, although it would be a merciful release.