The origin of this folk song is debatable. “The Girl I Left Behind Me” is claimed by both England and Ireland. It is said to date to the mid-1700s or even possibly back to the 1600s. The earliest known publication in print that lists the title and lyrics dates to 1791 in “The Charms of Melody,” Dublin, Ireland. The oldest known printed copy of the melody appears in “Himes Pocket Book for the German Flute or Violin”, Volume 3, also from Dublin, Ireland. There are numerous English, Irish and American versions of the lyrics. This song was also popular with early fiddlers and can be found in “Abel Shattuck’s Book”, circa 1801.

"Many versions of this standard exist, dating from the late eighteenth century. The melody is an authentic Irish folk tune from as early as 1660; Bunting included it in The Ancient Music of Ireland (1840) under the title, “An spailpin fanach.” As a song its earliest appearance was in 1758, when England was threatened by an invasion from France and an unknown Irish conscript is thought to have penned the lyrics.

The British (the English version is called “Brighton Camp”) officially introduced the song to America during the Revolutionary War. Later, soldiers of the Mexican War joked grimly about “The Leg I Left Behind Me,” and Civil War parodists in the same spirit of fun sang “I Goes To Fight Mit Sigel,” a dialect song which played on the stereotype of the Dutchman who, along with the Negro and the Irishman, enjoyed center stage at that time. Ever watchful for a hit, the irrepressible Harry Macarthy exploited the tune for “The Volunteer; or, It Is My Country’s Call” to stir up Southern patriotic sentiment. The song’s long history and numerous versions attest both to the sturdiness of its tune and to the universality of its sentiment." Source: https://ageofrevolution.net/born-in-battle-the-american-revolution-online/music/the-girl-i-left-behind-me

During the War of 1812, American soldiers are said to have heard a captured British prisoner singing the tune. The United States Army adopted the song and made “The Girl I Left Behind Me” into a march. The song still remains in the military’s repertoire and can be heard at West Point as part of a medley for the cadet’s final formation at graduation.

As can be seen from the references above, during The War Between the States both Union and Confederate composers wrote numerous lyrics favoring their side’s interpretation of the war.