From Movie "Gods and Generals"

YouTube - The Battle of Fredericksburg (Part 3: The Irish Brigade)

This is a very moving clip from the movie. The Northern Irish Brigade (this may be the 69th N.Y.) I didn't take the time to look it up, but they were units from N.Y., Penn, and Mass., some more famous. These boys were mostly new immigrants. They couldn't find shamrocks so they found sprigs of boxwood to put in their caps. Their green flag bears an Irish harp and the motto in Gaelic "Clear The Way" Gen. Meagher was their commander.

They weren't the only ones sacrificed by Burnside that December day. Four brigades were smashed against the stone wall.
Behind the wall was Gen. RR Cobb's Confederate Irish Brigade with the 24th Ga. in the center, Phillips Ga. Legion on his left and the 18th Ga on his right. The Regt also the 16th Ga. , Cobbs Legion supported by Cabell's Artillery. Repeated assaults came against the Georgans until they were low on amo.
Kershaw's S.Caroline Brig was moved in behind them. By this time the ranks stood five and six deep. I can't imagine what it would be like to shoot down your brothers knowing or believing they had been misled especially after what England had done to them.
The Georgia banners bore the same Irish Harp and a shamrock. Their northern opponents displayed the courage the Georgians would have expected. After the retreat the Southern boys took their hats off and cheered the valor of their brothers.
Many of the wounded froze to death that night.
Mark Levin in his new best seller "Liberty and Tyranny" tells why interference by the Fed. Govt brought about the war as Govt interference brings about so much other pain.