The organization of the Twenty-third Infantry Illinois Volunteers commenced under the popular name of the "Irish Brigade," at Chicago, immediately upon the opening of hostilities at Sumter. It served until the war had fully closed, and among the officers whom it was compelled to mourn as lost in the battle was its illustrious Colonel, James A. Mulligan, of Chicago, who fell while commanding a division of the Army of West Virginia at Kernstown, in Shenandoah valley, July 24, 1864, and perished while in the hands of the enemy, July 26, of three desperate wounds, received while at the head of his own Regiment to which he had galloped in the confident and justified expectation that he would be able to make it the steady rear-guard of an overwhelming rout, caused by the advance of all of Early's army upon an unsupported and meagre force.

The formal muster of the 23d was made June 15, 1861, at Chicago when the Regiment was occupying barracks known as Kane's brewery on West Polk street, near the river. From a barrack encampment on Vincennes road it moved July 14, 1861, to Quincy, Illinois, and thence, after afew day 5 encampment, to the arsenal at St. Louis. On the 21st of July it moved to Jefferson City, at a time when Colonel Jeff C. Davis was in command of the post. Daring the month of August it made various excursions into the surrounding country. Brigadier General Grant superseded Colonel Davis as commander of the post at Jefferson City, and on the 18th of September the 23d commenced & march of 120 miles on Lexington, Mo., where the first notable siege of the war of the Rebellion occurred. Lexington, reinforced by the 23d, which arrived on the evening of the 11th, became a post of 2,780 men, Colonel Mulligan commanding.  General Price with the Missouri State guards was marching upon the town, a convenient location near which Colonel Mulligan's command engaged actively in fortifying. The rebel advance under Raines with a battery of six guns assaulted the fortifications on the 12th but were repulsed. The post was then regularly invested by an army of 28,000 men with 13 pieces of artillery. For nine days the garrison sustained an unequal conflict, not alone against the vastly superior forces of the enemy but against hunger and thirst, for provisions, hastily gathered in from the surrounding country, were inadequate and the water supply wholly failed. No reinforcement appeared, nor was there promise or hope of any. On the 20th the most determined and systematic of the enemy's assault was made, and repeatedly repulsed, but in the afternoon it was determined to surrender. The killed and wounded of the Regiment numbered 107, while General Price officially reported his loss at 800.

The officers and men, with the exception of Colonel Mulligan, who was detained as a prisoner and accompanied Price in his march into Arkansas, were paroled. On the 8th of October the Regiment was mustered out by order of General Fremont, but upon the personal application of Colonel Mulligan, who had been exchanged for General Frost, General McClellan, then commanding the army, directed that its organization be retained and that it should be considered as continuously in the service from the date of its original muster. Reassembling at Camp Douglas in Chicago, the camp being commanded by Colonel Mulligan,  guarded the rebel prisoners there until June 14, 1862, when it was ordered it Harper's Ferry, Virginia. Its service thence forward was in both Virginias. From Harper's Ferry it moved to New Creek, Virginia. It was at Clarksburg, Virginia, in September and later at Parkersburg, in both cases saving the towns from the menace of Imboden. November 10, 1862, companies B, D and K under Major Moore attacked General Imboden on the South Fork of the Potomac, capturing forty prisoners and large supplies on the hoof. January 3, 1863, the Regiment made a forced march of 40 miles in 10 hours from New Creek to Morefield to the relief of the Union force there attacked by General Jones, who thereupon withdrew. In April, 1863, being then at New Creek, the Regiment was assigned to the 5th Brigade, 1st Division, 8th Corps, Colonel Mulligan commanding the Brigade and Lieutenant Colonel Quirk the Regiment. The Regiment moved to Grafton on the 25th of April, and Captain Martin Wallace, commanding Co. G, as a detachment in Greenland Gap, occupying a block house, had a spirited engagement with General Jones and did not surrender until the block house was in flames. April 25th the Regiment was engaged with Imboden at Phillippi.  In 1863 the Regiment was on the flank of Lee in his retreat from Gettysburg, and had an engagement with Wade Hampton at Hedgeville. Having re-enlisted as veterans at New Creek in April, 1864, the Regiment was reorganized at Chicago and the month's furlough having expired returned to Virginia.

During the month of July, 1864, the Regiment participated in the following engagements: 3d, Leetown; 5th to 7th, Maryland Heights, Md.; 17th to 20th, Snicker's Gap. Va.; 23d and 24th, Kernstown, Va., where Colonel Mulligan was killed. In the battle of Kernstown on the 24th, the Regiment lost in killed and wounded about one-half of those engaged therein.

From early in August, 1864, to December 25, 1864, during which time General Sheridan was in command of the Shenandoah Valley, the Regiment was actively engaged therein, and took part in the following battles and skirmishes:  Cedar Creek, August 12th to 16th, Winchester, August 17th; Charlestown and Halltown August 21st to 28th; Berryville September 3d: Opequan Creek, September 19th; Fisher's Hill, September 21st and 22nd, Harrisonburg, October---; Cedar Creek. October 13th; Cedar Creek, October 19th. About December 30th, 1864, the Regiment was transferred to Army of the James, and during January, 1865. was in front of Richmond, and was afterwards assigned to the defenses of Bermuda Hundreds. March 25, 1865, rejoined Twenty-fourth Army Corps north of the James River, and thence moved to the left as far as Hatcher's Run, where was engaged March 31st and April 1st, and on April 2d assisted in the assault and capture of Fort Gregg in front of Petersburg, and thereafter took part in the pursuit of Lee's Army until the surrender thereof at Appomattox C. H., April 9, 1865.

In the months of January and February, 1864, while stationed at Greenland Gap, W. Va., First Lieutenant John J. Healy, as special recruiting officer, re-inlisted about 300 of the Regiment as veterans, and in May following they came to Chicago on thirty days' furlough, as the Twenty-third Regiment Illinois Veteran Volunteers.

In August, 1864, the 10 companies of the Regiment, then numbering 440, were consolidated into five companies, and was designated "Battalion Twenty-third Regiment Illinois Veteran Volunteer Infantry,' and Lieutenent Colonel Simison assigned to command. In March, 1865, Colonel Simison returned to Illinois, leaving Captain P. M. Ryan in command, to have five new companies assigned by the Governor to fill the Regiment, and in this he was successful, but the new companies did not meet the Veterans until the surrender of Lee.

The Regiment was thanked by Congress for its part at Lexington, and was authorized to inscribe Lexington upon its colors.

Two medals, authorized by Congress, were given members of the command for gallant conduct. They were bestowed upon Private Creed, Company C, who, at the battle of Fisher's Hill, knocked down a rebel color-bearer and captured his flag, and Private Patrick Hyland. Company D, who was the first soldier to scale the rebel works at Ft. Gregg, Petersburg, April 2, 1865.

The orders for consolidation after the death of Colonel Mulligan, are appended. Lieutenant Colonel Quirk and Major Moore retired after the veteran re-organization.

The re-enlisted men and recruits of the Twenty-third Regiment Illinois Volunteer Infantry will be formed into companies, of the legal maximun standard, and the following officers are selected and will be retained in the service to officer them:

Captain S.A. Simison, Captain Harry Pease, Captain Martin Wallace, First Lieutenant T. D. McClure, First Lieutenant John Healy, First Lieutenent Patrick Ryan, Second Lieutenant Bartholomew Quirk, Second Lieutenant James Fincan, Second Lieutenant Julius Fletcher, Second Lieutenant S. S. Allen, Assistant Surgeon John S. Taylor.

These companies will be formed under the direction of Colonel J. Thoburn, commanding First Infantry Division, who will assign the above named officers to their respective companies--one of the Second Lieutenants named being assigned to the command of the detachment remaining after three (3) full companies are formed. The remaining officers and men whose term of service has expired. and who have not re-enlisted, will proceed to Chicago, Ill., to be mustered out.

     By order Brevet Major General CROOK.

VII. So much of Par. 9. Special Orders No.157, current series, from these Headquarters, as reads, "after three (3) full companies are formed," is hereby amended to read, "after four (4) full companies are formed."

In accordance with Special Orders Nos. 157 and 159, Department West Virginia, the Twenty-third Regiment Illinois Volunteer infantry is consolidated, as follows:

I. Twenty-four (24) men of company A and seventy-four (74) men of company B are consolidated into one company, to be designated "Company B." The following officers are assigned to command: Captain Harry Pease, Second Lieutant James Fincan.

II. Fifty (50) men of com p any C, forty-two (42) men of company D, three (3) men of company A, and three (3) men of company H are consolidated into one company, to be designated "Company D."  The following officers assigned to command: Captain Samuel A. Simison, First Lieutenant Thomas D. McClure.

III.       Twenty-one (21) men of company E, fifty-two (52) men of company G, and twenty-five (25) men of company H, are consolidated into one company, to be designated "Company G." The following officers are assigned to command:  Captain Martin Wallace, First Lieutenant John J. Healy, Second Lieutenant Julius E. Fletcher.

IV. Forty-six (46) men of company I, and fifty-two (52) men of company K to be consolidated into one company, to be designated "Company K." The following officers are assigned to command: First Lieutenant Patrick Ryan, Second Lieutenant Bartholomew Quirk.

V. Thirty.two (32) men of company F, and four (4) men of company I to be consolidated into one detachment, to be designated "Detachment F." The following officer is assigned to command: Second Lieutenant Stewart S. Allen.

The consolidated organization will be called "Battalion Twenty-third Regiment Illinois Veteran Volunteer Infantry."

All men belonging to the Battalion Twenty-third Illinois Volunteers, not already enumerated in orders from these Heaaquarters, and any that may be recruited, will be assigned to "Detachment F," until it becomes a maximum company.

    By order Colonel JOSEPH THOBURN.

SIR-I have the honor to inform you that the following commissioned officers were mustered out of the service at this office, on the dates set opposite their names, by reason of expiration of their terms of service:
Captain Patrick McDermott, company A, Twenty-third Illinois Volunteers, September 14, 1864.
First Lieutenant John A. Daily, company A, Twenty-third Illinois Volunteers, September 14, 1864.
Second Lieutenant Martin J. Russell, company A, Twenty-third Illinois Volunteers, September 14, 1864.
First Lieutenant Edward S. Murray, company B, Twenty-third Illinois Volunteers, September 14, 1864.
Second Lieutenant John Brown, company C, Twenty-third Illinois Volunteers, September 14, 1864.
First Lieutenant P.O. Kane, company F, Twenty-third Illinois Volunteers' September 14, 1864.
First Lieutenant Anthony McBriarty, company H, Twenty-third Illinois Volunteers, September 14, 1864.
Captain James J. Fitzgerald, company I, Twenty-third Illinois Volunteers, September 19, 1864.
Surgeon Patrick Gregg, Field and Staff, Twenty-third Illinois Volunteers, September 10, 1864.
Second Assistant Surgeon C. W. Stinson, Field and Staff, Twenty-third Illinois Volunteers, September 14, 1864.

VI. So much of Special Orders No.157, Par. 9, as directs the retention in the service of First Lieutenant Thomas D. McClure, Twenty-third Illinois Volunteers, is hereby revoked. Lieutenant McClure having been on detached duty at the date the term of service of the Regiment expired, it is probable he was mustered out at that time.

In compliance with General Order No.86, War Department, Adjutant General's Office, series of 1863, the letters of the companies of the Battalion Twenty-third Illinois Veteran Volunteer Infantry are changed as follows: