Answer: The Irish Brigade.

From the start of the Civil War, Irish men served on both Union and Confederate lines.  It was not until 1862 that a specifically Irish brigade was organized by Captain Thomas Francis Meagher, first recruiting troops in New York and then in Philadelphia.  The 116^th Pennsylvania Infantry Regiment joined the ranks of the Irish Brigade in the summer of 1862 at their headquarters at Seventh and Market Streets under the direction of Colonel Dennis Heenan. 

William McCarter was born in Derry, Ireland in the 1840s and lived in Philadelphia during the Civil War.  He joined the 116^th Pennsylvania Infantry Regiment in August 1862 and fought in 26 battles, mostly in Virginia, including Fredericksburg, Chancellorsville, and Spotsylvania.  McCarter documented his military service in detail with the 116^th Regiment in his manuscript titled, “My Life in the Army,” a rare glimpse into an Irish-American Civil War soldier’s experience. “Every man in the Brigade wore a sprig of green box-wood in his cap,” McCarter noted in the first volume of his manuscript.