I got this from the liner notes of Far From the Shamrock Shore by Mick Maloney:

"This song was written by Dan Emmet(1815-1904)one of the most famous Irish American performers in minstrelsy. It was written on a rainy day in New York city in 1859, probably adapted from African American antecedents. It was originally designed to be a minstrel walk-around closing the show, but he watched, astonished as it became the anthem of the Confederacy during the Civil War(which is why it inflames the emotions of so many people).

Brían