Short answers, pulled from the story.
Patrick Gilmore wrote the lyrics to When Johnny Comes Marching Home during the American Civil War. The song's first publication was deposited in the Library of Congress on the 26th of September 1863. Words and music were credited to Louis Lambert instead of Gilmore himself.
The song's first publication was deposited in the Library of Congress on the 26th of September 1863. Copyright remained with Henry Tolman & Co. of Boston. The melody appeared around the 1st of July 1863 as the music to the drinking song Johnny Fill Up the Bowl.
Why Gilmore used a pseudonym remains unclear but popular songwriters often employed pseudonyms to add romantic mystery to their songs. He later acknowledged that the music was not original and described it as a musical waif he heard somebody humming in the early days of the rebellion. He took a fancy to it and wrote it down before dressing it up and giving it a name.
Gilmore is said to have written the song for his sister Annie who prayed for the safe return of her fiancé Union Light Artillery Captain John O'Rourke. They were not married until 1875 though they may not have been engaged in 1863. The lyrics expressed longing for friends and relatives fighting in the war.
Confederate forces adopted the same melody for their own purposes while Union soldiers sang it while marching through Southern territory. The song became immensely popular during the conflict and was sung by both sides of the American Civil War. It served as morale booster across battle lines with broadside illustrations showing Zouave companies marching with drums beating.