I've ransacked a number of reliable databases for the alleged "American Civil War" stanza of "The Minstrel Boy" in the DT without finding any reference to it before 1997, more than 130 years later.

The DT doesn't say the stanza was added *during* the Civil War, but that hasn't stopped various websites from asserting that it was!

The diction strikes me as post-1918, if not later.