It never happened... the Confederate Irish unit was on another part of the line, On the far right IIRC. I believe the only time the two all Irish units actually exchanged fire was during the Battle of Antietem... though I'm not certain.
That most if not all of the men who fought in the Civil War were fighting men is not a question that needs answering. Many friends & family fought each other. Particularly in the border states. The lost causers would have people believe that the lines were every clearly drawn, they weren't. Historically Kentucky was better than 80% Union loyal, part of Tennesee, Eastern Tennesee in particular was heavily pro Union. Northern Alabama and parts of Louisiana as well had staunch pro Union sentiment. However, on the other side of the coin Illinois had a VERY large pro confederate bias and there were pockets in Iowa, Indiana and New York as well. So the proportion of friends and family doing their best to kill each other was far higher than many are willing to admit.
All in all I was horribly disapointed w/ Gods & General. I think I would have prefered to see the Antietem footage than the Fredricksburg. The movie was supposed to be about Jackson, while his role at Fredricksburg was important it was no where near as pivotal as at Antietem. Antietem showed all of his qualities far better than Fredricksburg. My own thought of why Fredricksburg was included instead of Antietem is that Fredricksburg was a decisive victory for the CSA and the director and Mr Turner didn't want to show the Union in anything but a poor light. Frankly I think they relished the scenes of Union soldiers falling en masse which they did a fine job of. As I've said... I'm just glad I know who surrendered at Appomotax.
I think I'm glad we're not going to see the next installment...
As a cinematic experiance it was a nice work, as a historical interest maker... it was abysmal. I would have been much happier if it had been just half the movie that Glory or Gettyburg were. Anything Hollywood puts out is guaranteed to have its historical shortcomings and all things considered Gods & Generals had less than most. I just wish a truly talented director the caliber of Spielberg would give us something about the Civil War.