The 28th Massachusetts Volunteer Infantry was the second Irish regiment formed in Massachusetts during the Civil War (the 9th Mass. preceded them). When the regiment was recruited enlistees were told that the 28th was to be part of the Irish Brigade being organized by Brigadier General Thomas Francis Meagher. Three NY regiments were already in the Irish Brigade, the 63rd, 69th, and 88th.
Oddly enough, the War Department assigned the 29th Massachusetts to the Irish Brigade instead of the 28th. The problem was that the 29th was not an Irish regiment at all!
The 28th was sent to Hilton Head, South Carolina where it was used in coastal operations. It was not assigned to the Army of the Potomac until July 1862, but even then it was not placed in the Irish Brifgade. Instead they were incorporated into the IX Corps under Burnside and fought in that corps at 2nd Bull Run, Antietam, and the other summer and fall battles of the Army of the Potomac.
The 28th only became part of the Irish Brigade in November 1862. Unfortunately it joined the Brigade immediately before the bloody Battle of Fredericksburg.