After Chancellorsville, the new Brigade Commander, Col Patrick Kelly of the 88th, formed these "core" NY regiments, now together only numbering 220 effectives, into a single battalion under the flag of the 88th.

After Bull Run, Thomas Francis Meagher, the Captain of Company K, applied to have the 69th New York Volunteer Militia reorganized into Federal service as the core unit of a larger brigade composed predominantly of Irish immigrants.

A visible participant in the failed Rebellion of 1848, he was afterward tried and sentenced to death (commuted to life imprisonment in Australia, but he escaped to San Francisco CA).

Gen. Meagher's battle sword, made by Tiffany & Co, is now on permanent exhibit at the Fredericksburg and Spotsylvania National Military Park.

The Irish Brigade distinguished itself from the rest of the Army of the Potomac by Meagher's insistence on arming the eight line companies of each NY regiment with Model 1842 smoothbore muskets, an obsolete weapon that was largely phased out during 1862, because he wanted his men to be able to fire buck-and-ball shot (a .69 caliber musket ball with four smaller balls), which produced a shotgun effect in close-range combat and could not be used with rifles.

The three original New York regiments carried Model 1842 muskets all through the AoP's campaigns and battles in 1862–63, using buck-and-ball shot with deadly effect in the Sunken Road on September 17, 1862 at Antietam and in the Wheatfield on July 2, 1863, at Gettysburg.

The 28th Massachusetts (which joined in October 1862) had Enfield rifles and were with the 6 Company, NY "Light Battalion" often detailed for skirmishing duty.

Meagher assumed his brigade would perform most fighting at close range where smoothbores were effective and his officers generally agreed.

There are relatively few complaints on record from the enlisted men about their outdated muskets, although one veteran of the 88th New York recalled that "we were sometimes at a disadvantage because of the short range" and that he had to pick up a discarded rifle from the field at Antietam to deal with Confederate skirmishers.

Despite their divergent backgrounds, the 29th Massachusetts and the rest of the brigade fought well together, earning plaudits for hard campaigning during the Seven Days Battles; most notably at Savage's Station, Glendale, and Malvern Hill.

After Malvern Hill, the Army of the Potomac languished at Harrison's Landing on the Peninsula and Meagher gained permission to recruit in New York to replenish the brigade's losses.

While other units were transferred to northern Virginia during the summer of 1862 to fight under Gen. John Pope, the Irish Brigade remained on the Peninsula with Gen. George B. McClellan.

The brigade's new recruits, approximately a tenth of the number that Meagher had hoped to raise, joined the unit at Tennallytown, Maryland, in time to march in pursuit of the Confederates.

In May 1863, the brigade sustained further casualties at the Battle of Chancellorsville; Meagher repeated his request to recruit replacements, was denied, and resigned his commission in protest.

Leading up to the Battle of Gettysburg, the brigade recovered several hundred of its injured from Fredericksburg and was able to field nearly 600 men - in reality, barely at regimental size.

The lineage of the Irish Brigade has been officially assigned to "Fighting 69th" of the New York National Guard, which is the only currently active military unit that formed part of it.

For bravery displayed in Lorraine, Champagne-Marne, and Meuse-Argonne, the Medal of Honor was awarded to regiment members, including William Joseph Donovan and Richard O'Neill.