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Also known as "When Johnny Comes Marching Home Again", this song which dates from the American Civil War was sung by both sides, as was the slightly earlier "Lorena". The lyrics were written by the Irish-born Patrick Gilmore (1829–92) who served with the Union Army. The bandmaster and composer took the melody from "Johnny I Hardly Knew Ye", another war-related song, but one that has a somewhat dissimilar message.
Gilmore published his song under the pseudonym Louis Lambert; it is better known than its precursor, and has in some respects become a standard; it has certainly been recorded widely and in many variants.
Among others, Guns N' Roses included part of it in "Civil War", and a few bars were used at the end of "Out In The Fields". (thanks, Alexander Baron - London, England, for above 2)