We sang what was likely a sanitized version of this well-known folk song in elementary school in 1947-8 or so. It was a pretty innocuous tune, as we learned it, and we were told it did date from the Civil War. Being an Army veteran, I can attest to the G.I. penchant for "politically incorrect" adaptations of popular songs. I'm sure it was no different in the 1860's. Check out the "Wikipedia" snippet, attached:

"The Girl I Left Behind" also known as "The Girl I Left Behind Me" is a long-standing popular folk tune. According to Theodore Ralph, it was known in America as early as 1650, under the name "Brighton Camp". In Ireland it was known as "The Rambling Labourer" and "The Spailpin Fanach" and was first published in Dublin in 1791. [1]

It has many variations and verses. Here is one example:

All the dames of France are fond and free
And Flemish lips are really willing
Very soft the maids of Italy
And Spanish eyes are so thrilling
Still, although I bask beneath their smile,
Their charms will fail to bind me
And my heart falls back to Erin's isle
To the girl I left behind me.