This unit was the "Irish Brigade," a unit in the Union army. One of its Officers was Thomas Francis Meagher a Young Ireland leader in the 1848 uprising at Ballingarry. A number of the Young Irelanders escaped to America and joined the Officer Corp of this regiment. It was composed mostly of Irish immigrants, its core was the 69th New York Infantry.

The men in the photograph were Catholic chaplains from the University of Notre Dame in Indiana.
Seated, left to right: Captain J.J. McCormack, James Dillon, C.S.C. (Holy Cross order), and William Corby, C.S.C.
Standing: Patrick Dillon, C.S.C., and Dr. K. O'Hanlon

Fr. Corby is memorialized in bronze on the site of the Battle of Gettysburg,
a sculpture showing him giving general absolution to the Irish Brigade
before they entered the battle on July 2, 1863.

Both Corby and Patrick Dillon served as presidents of the University.

Appreciation to Stephen Moriarty Notre Dame University for this info. (B.M)