I spoke with Ted Deddens of TedToy at the Indianapolis show and got a similar impression -- that the figures in this series really don't represent any particular action.
However, I think the fight at South Mountain may be the most likely. Below is an excerpt about the brigade published in 1866:
The brigade advanced, and found the enemy posted in the woods, and behind stone walls, and drove them before them until he was reinforced. In order to protect the right flank, Lieutenant Colonel Bragg [6th Wisconsin] entered the woods on the right, and deployed his regiment to the right of the Seventh. The Nineteenth Indiana, supported by the Second, deployed, and swung around parallel to the turnpike, and took the enemy in flank , getting a raking fire upon him, as he lay behind the stone walls.
Thus, it appears that the Iron Brigade did assault Confederate troops formed behind stone walls at South Mountain. I have not read any of the unit histories mentioned earlier in this thread, so I'll ask those who have -- How does the above description of South Mountain compare with what you have read?