The unit underwent additional training at Fort Columbus in New York Harbor before being dispatched in early 1862 for its first active duty assignment.

[2] Known for their distinctive Tiffany-embroidered green flag and Gaelic war cry, "Faugh a Ballagh" (Clear the Way), the Irishmen of the 28th Massachusetts saw action in most of the Union Army's major eastern theatre engagements – Antietam,[3][4] Fredericksburg, Chancellorsville, Gettysburg, the Overland Campaign, and the siege of Petersburg – and were present for Gen. Robert E. Lee's surrender to Gen. Ulysses S. Grant at Appomattox Court House.

But a sufficient number re-enlisted by January 1, 1864, to justify the continuation of the regiment as a five-company battalion of "veteran volunteers" until the end of the war.

Although resented and protested by the other officers of the regiment, Byrnes served with distinction and was wounded on June 3, 1864, while leading the Irish Brigade at the Battle of Cold Harbor.

The surviving veterans of the regiment marched in Washington, D.C., during the Grand Review of the Armies that celebrated the war's conclusion, then traveled home to Massachusetts, where they were paid and discharged from the service at Readville in June 1865.