William Howard Irwin was born in Mifflin County, Pennsylvania in 1818. He enrolled with the class of 1840 at Dickinson College in Carlisle, Pennsylvania in the fall of 1836. He was elected as a member of the Union Philosophical Society but left the College after two years to study law at home in Lewistown where he was called to the bar in 1842.
He practiced in Lewistown until February 1847 when he and a friend traveled to Washington D.C. to request and win commissions in the 11th U.S. Infantry. They returned to recruit a unit from Mifflin County for service in the Mexican War. This company, known locally as the Juniata Guards, became Company D, 11th Infantry and left Lewistown on March 25, 1847. Giving the main speech seeing off the troops was Irwin's fellow lawyer, Lewistown native, and Dickinsonian, James K. Kelly. On September 8, 1847, with the 11th serving in Pillow's Division at the Battle of El Molino del Rey, Irwin was seriously wounded at the head of his company. Following recuperation, now breveted as a major for his bravery, he returned to Lewistown with his company in August, 1848.
He returned to practice until the outbreak of the Civil War when he again went into uniform. He enlisted as a private immediately after the firing on Fort Sumner, serving in the "Logan Guards" helping to secure the national capital from sudden attack. He was named soon after as the colonel of the 7th Pennsylvania Volunteers, a ninety days enlistment unit that participated in the early push down the Shenandoah Valley in June and July 1861. Following the mustering out of the 7th, he assisted in the raising and organizing of Pennsylvania units until he was appointed colonel of the 49th Pennsylvania Infantry in late 1861. During the training of that unit, he was involved in controversy when several officers under his command filed charges and he was tried under court martial early in 1862 for drunkenness and "conduct prejudicial to good order and military discipline." He was acquitted on the first charge but convicted on the second, which drew him an inconsequential suspended punishment. He went on to lead the 49th with distinction in the Peninsula Campaign and was appointed as a brigade commander in the 2nd Division of the VI Corps for the advance into Maryland that culminated at Antietam in September 1862. His 3rd Brigade fought well during the battle and Irwin was commended. The next major engagement for the unit was at Fredericksburg when, on April 29, 1863, crossing the river in the pontoons under fire, Irwin was wounded in the foot. In October, 1863, increasing troubled by illness and wounds and suffering from mental exhaustion, he was declared unfit for field service and he resigned his commission. Following the war, he was named as a brevet brigadier general for his service and his conduct at Antietam.
He had before the war married a widow named Mitchell. His stepson, William Galbraith Mitchell served with him and ended the war himself as a brevet brigadier general. Following the war, Irwin practiced law, and was involved with coal and railroads. Increasingly unstable, he died from exhaustion in the Central Kentucky Asylum in Anchorage, Kentucky on January 17, 1886. He was sixty five years old.