Short answers, pulled from the story.
Irish-American bandleader Patrick Gilmore wrote the lyrics during the American Civil War. He published the song in 1863 under the pseudonym Louis Lambert, with copyright held by Henry Tolman and Co. of Boston.
The first publication was deposited in the Library of Congress on the 26th of September, 1863. The lyrics also appeared that year in Erastus Beadle's Beadle's Dime Song Book No. 15, on pages 18 and 19.
Gilmore acknowledged in an 1883 Musical Herald article that he did not compose the tune. The melody had appeared earlier as the music for the Civil War drinking song "Johnny Fill Up the Bowl," and scholars have traced resemblances to tunes dating back to around 1630 or earlier.
In 1942, Glenn Miller recorded the song on RCA Bluebird Records with Tex Beneke, Marion Hutton, and The Modernaires. Guy Lombardo and The Andrews Sisters also recorded it that year on Decca Records.
The two songs share the same tune, but "Johnny I Hardly Knew Ye" was not published until 1867 and originally had a different melody. "When Johnny Comes Marching Home" was so popular in England and Ireland that it eventually influenced the creation of the related anti-war song.
In the John Ford western The Horse Soldiers, the last line was changed to "And we'll all raise hell when Johnny comes marching home." English pop singer Adam Faith sang a version for the 1960 British crime thriller Never Let Go, arranged and conducted by John Barry, and a single release reached No. 5 in the UK Singles Chart.