Wrong. Or at least, the assumption that an Irishman fighting for the Confederacy is wrong.
I don't know about just-off-the-boat Irishmen enlisting, but just from listening to personal anecdotes of Irish-American people who had ancestors fighting for the Confederacy, it's lunacy to assume they wouldn't have. Remember, we may look at the Civil War and pronounce that it was about slavery and nothing else, but for many (most) Confederate soldiers (those who enlisted and who were conscripted), the idea of fighting for land, freedom, and against "them Yankees" were heartily embraced by even the poorest members of the South, and most Confederate troops were subsistence farmers.
It just really rattles me when people attempt to impose a revisionist view of the Civil War, where anyone who didn't fight for the Union was a terrible, horrible, no-good human being, but that's just bogus. Even though my Irish ancestors were still toiling in Ireland in the 1860s, there were still plenty of them who fought for the Confederacy. WHAT OF IT?

Two points I'd like to mention here. First, concerning Richard Harris as the "unlikely Cracker" as Charlton Heston referred to him. Harris was personally fond of the American West and made several westerns in his career, none of which did he attempt to force any type of American accent. Secondly, referring to the movie not having an end as mentioned by a poster earlier, I felt that the fight at the river with the French made the movie. At first Dundee's command was disjointed, then divided at the point of O. W. Hadley's killing. Then later the team "gelled", put in current sports lingo at the rescue of Dundee, then again at the destruction of Charriba and his group. Then the command was further cemented during the fight with the French, as symbolized by Tyreen's killing of the Frenchman who had seized the US flag after killing the US flag bearer. The entire scene was choreographed perfectly. My opinion only here of course, and I must add that this is one of my favorite movies as well. Viva Dundee! (even if he was a Yankee).
"check the imdb cast list before asking who portrayed who in movies please"

Too bad Sam didn't spend more time writing final act for the film. The big problem is that the film just falls apart in its last half and has no ending.
This is an old thread; but no, I say the BIGGEST problem was the studio's 11th-hour decision to cut both a third of the production budget and a third of the shooting schedule while still intending to have it meet the original start date. Everything else stems from that folly.
Throughout production, Peckinpah filmed by day while rewriting the script at night, trying to stay ahead of the next day's shooting. And aside from the unrealistically shortened schcedule, another side of this disaster came from Peckinpah's late discovery that the original writer (Harry Julian Fink, I believe) had failed to make the extensive revisions which the director had ordered weeks earlier.
Therefore Peckinpah, stuck directing a strenuous production while simultaneously rewriting it overnight (much like Joseph L. Mankiewicz was forced to do on CLEOPATRA), reacted to the constant hovering threat of studio interference by developing an unfortunate identification with the Dundee character. That led to Peckinpah's stubborn insistence that the Major had not only to survive, but to triumph.
Most great films deserve a more appreciative audience than they get
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True story BTW. See Heston's "An Actor's Life" journal where he comments in the early stages, "Sam's convinced he'll never get the accent. I concede he makes an unlikely cracker." Then the next day at a meeting, Peckinpah decided to throw out Harris's accent and make him an "honest Irishman" and at the same meeting Peckinpah had Heston shave off the Cavalry beard he'd spent six weeks growing for the part deciding he wanted to have Dundeed clean-shaven up to the point of his wounding.
Too bad Sam didn't spend more time writing final act for the film. The big problem is that the film just falls apart in its last half and has no ending.

"The hell with it, just make him an Irishman fighting for the south."
When I'm gone I would like something to be named after me. A psychiatric disorder, for example.

They made Harris an Irishman in the film for a simple reason. After they cast him in the part of Tyreen, he kept trying to get a handle on a southern accent and finally when it became clear that he was never going to get it right, Peckinpah decided "The hell with it, just make him an Irishman fighting for the south."

Well if he was an Ulster Protestant he certainly didn't have THAT accent!

Most of the Irishmen who fought for the South were actually Ulster Protestants, thus distinctly different from their primarily Irish Catholic counterparts fighting for the Union.
"If you cant say something good about someone, sit right here by me."

I got the impression that he was from the Anglo-Irish aristocracy, and as such wold not have had the archetypal "Paddy" accent.

Actually cashiered -and Dundee, his socalled friend,voted to cashier him over an affair of honour.Disgusting!

Doesn't he only fight for the South because he was jailed by the North for slaying an officer in a duel at Westpoint?

Thomas Meagher's Irish Brigade is the best example of what you say, Hancock, as they fought with unmatched glory for the North in such battles as Antietam and Gettysburg. Cleburne did fight for the South, though, his story is also fictionalized well as a background character in William Safire's "Freedom," though I recall Safire kills him off at the Battle of Stone's River (with an explanation afterwards in his terrific "Underbook" of how it really went down.)
It's interesting for Dundee's ragging on him as an Irish potato farmer that Harris doesn't play Tyreen with an Irish accent. He was an Irishman in real life, but here his voice suggests a more Mid-Atlantic reading.

Don't forget Patrick Cleburne, the great Irish Confederate general who fell at the Battle fo Franklin in 1864.
However, the majority of Irishmen did fight for the North, so it's something of a valid point.
"We're all made of the same CLAY, you know!? Clay!
CLAY!
"