A Forum search shows more threads discussing this song than I can review easily.

The central facts of the matter, however, appear to be as follows:

"When Johnny Comes Marching Home Again" was copyrighted by Union Army bandmaster Patrick Gilmore (using the pseudonym "Louis Lambert")in 1862 or '63. It became popular instantly. Gilmore, however, claimed credit only for the upbeat, celebratory lyrics; he claimed, somewhat enigmatically, that he had heard the tune hummed by an African-American youngster in New Orleans. There has been much conjecture about what tune Gilmore may have heard.

As Malcolm reminds us, sheet music of "Johnny, I Hardly Knew Ye," with words and music credited to James B. Geoghegan, were published in London a few years later, ca1867. I've seen a copy of this publication, and while the lyrics are the familiar "JIHKY" words, the tune is clearly not Gilmore's or anything much related to it.

There is absolutely no reason to assume that the (tuneless) broadsides of "JIHKY" are any older than Geoghegan's song.

In his discussion of the song, written many years later, Herbert Hughes claimed to have remembered the song from about 1867. He'd thought it was a new song until "an old fisherman" told him at some point, as an aside, that it was much older. The fisherman's claim is the only evidence at all for thinking "JIHKY" to have preceded Gilmore's "Johnny."

And I believe that's all anybody knows about the relationship between the two songs. Any number of conjectures about "lost originals" are possible but go nowhere. The preponderance of evidence points to Gilmore's song as the original and Geoghegan's as the parody.

The likely equivalency of "Sulloon" (or "Siloam") with "Ceylon," and the fact that Irish troops were stationed there before the American Civil War of 1861-65, do not together prove that "JIHKY"
predates the publication of "WJCMHA." The only thing that would do that would be a pre-Civil War appearance of "JIHKY."

Nobody has unearthed one, or even a first-hand recollection of one.
Hughes's anecdote pushing the song back to the early 19th Century remains hearsay unsupported by any other testimony.

"JIHKY" has been recorded so frequently since the '50s that I've met people who mistakenly think that *its* title is "When Johnny Comes Marching Home Again," having never heard Gilmore's lyrics.