Shall my soul pass through Old Ireland is definitely from the time of Terence MacSwiney's death. I'd be very surprised if it's a rewrite of an American Civil War Song (though the tune's been around a bit). Which suggests that the Rebel Soldier is a modern song based on the MacSwiney song.

Of course it could still be drawing on a bit of verse from the American Civil War, rejigged to fit the tune and shape of the Irish song.