The 23rd Illinois Volunteer Infantry Regiment, known as the "First Irish" or "Irish 'Brigade'", was an infantry regiment that served in the Union Army during the American Civil War.

Initially assigned to garrison the town of Lexington, Missouri, the regiment surrendered to a much larger force of pro-secession Missouri State Guard commanded by State Guard Major General Sterling Price at the Siege of Lexington in September 1861.

After being paroled, General John C. Fremont (commanding the Department of Missouri) had the 23rd Illinois mustered out of service, but in December General George McClellan (now supreme commander in chief of all Union armies) had it restored.

After being reconstituted, the regiment left camp in the spring of 1862 for western Virginia, where it spent most of the rest of the war stationed there.

In 1864, it joined General Philip Sheridan's army in the Valley Campaign.

At the end of the year, the 23rd headed to the Richmond area for the Siege of Petersburg and was present at Appomattox.

Reassembled at Chicago and guard prisoners at Camp Douglas until June 14, 1862.

Attached to R. R. District, Mountain Department, Harper's Ferry and New Creek to July, 1862.

R. R. District, 8th Army Corps, Middle Department, to September, 1862.

New Creek, Va., Defenses Upper Potomac, 8th Army Corps, Middle Department, to March, 1863.

Kelly's Command, Reserve Division, West Virginia, to July, 1864.

1st Brigade, 3rd Infantry, Division West Virginia, to July, 1864.

3rd Brigade, 1st Infantry Division, West Virginia, to December, 1864.

Raid on Baltimore & Ohio Railroad between Bloomfield and Piedmont May 5 (Non-Veterans).

Duty in trenches before Richmond and on the Bermuda Hundred front until March 27.

The regiment suffered 4 officers and 50 enlisted men who were killed in action or who died of their wounds and 2 officers and 93 enlisted men who died of disease, for a total of 149 fatalities.