Another of Illinois' courageous immigrant groups was the Irish who had settled mostly in Chicago. On April 20, 1861, the Chicago Tribune announced a rally of Irishmen wishing to serve their adopted nation in the war. That evening thirty-two men were recruited after hearing James A. Mulligan speak. Plans for acquiring equipment were made and word of the recruiting was sent to neighboring Irish communities. In a few days more than 1,000 men had joined the brigade.

Once organized, their services were offered to the government but rejected, since Illinois had already filled its quota. Refusing to accept this, Mulligan journeyed to Washington to ask that they be accepted for service. On May 17, the War Department complied, and on June 15, 1861, Chicago's "Irish Brigade" officially became the 23rd Illinois Infantry under Colonel James Mulligan.

The regiment's first battle earned them fame. On September 18 at Lexington, Missouri, they were attacked by Confederate troops. At first, they pushed the enemy back, but as the battle wore on and no reinforcements came, the enemy gained the advantage. Hoping against hope for relief, the 23rd held out for two days, but no help came. When it was forced to surrender, Mulligan alone was taken prisoner but was released in October. He returned to Chicago on October 30 where he was received as a hero.

Restored to active duty by early 1862, the Irish Brigade was assigned guard duty at Camp Douglas, a prisoner-of-war camp. Mulligan, in charge of the prison, reformed the poor management system at the camp and granted the prisoners much more humane treatment than had his predecessor. In June the regiment was moved to Maryland for field service and played a small role in the victory at Antietam. Next it was ordered to western Virginia and given the thankless task of battling rebel guerrillas. It spent two years warding off raids, ambushes, and night attacks.
http://www.lib.niu.edu/1994/ihy940232.html