Bernard Irwin was born on June 24, 1830, in County Roscommon, Ireland. He immigrated to the United States with his parents in the 1840s, and he enlisted in the 7th Regiment of the New York National Guard while attending college in 1848, and served until his honorable discharge in 1851. He graduated from New York Medical College in 1852, and served as a surgeon and physician at the State Emigrant Hospital on Ward's Island, New York, from 1852 to 1855. In 1855 he was appointed an acting assistant surgeon in the U.S. Army and served at Fort Columbus in Corpus Christi, Texas, and he was appointed to the regular corps as an assistant surgeon beginning on August 28, 1856. His next assignment was as an assistant surgeon at Fort Union, New Mexico, followed by service at Fort Defiance, Arizona, and during this time he was in the field in operations against hostile Navajo and Apache tribes. Lt Irwin was assigned to Fort Buchanan, Arizona, from December 1857 to November 1861, and during this time he participated in the rescue of Lt George Bascom, who had been captured with sixty men by Apache indians in January 1861. For his heroism on this mission, Lt Irwin would later be awarded the Medal of Honor for the earliest action in which the Medal would ever be awarded; 13 February 1861. Capt Irwin moved with the 7th Infantry from Fort Buchanan to Jefferson Barracks, Missouri, in November 1861, and in early 1862 he was appointed Medical Director of General Jeremiah T. Boyle's brigade and then Medical Director of General William Nelson's division in the Army of the Ohio. During the Battle of Shilo in April 1862 he was credited with inventing the tent field hospital, the model for which later field hospitals were based. After participating in the siege of Corinth, Mississippi, Irwin transferred to the Army of Kentucky in August 1862, and was captured by Confederate forces during the Battle of Richmond, Kentucky, in late August 1862. Irwin was promoted to Major during the time he was held captive, and was released by the Confederate Army in October 1862. He was then appointed Medical Director of the Army of the Southwest, and participated in the White River Expedition in January 1863. Maj Irwin next served as a surgeon at the Army hospital in St. Louis, Missouri, from February to March 1863, followed by service as surgeon and superintendent at the Army hospital in Memphis, Tennessee, from March 1863 to July 1865, having received a brevet promotion to Lieutenant Colonel and then Colonel in March 1865. Col Irwin served as post surgeon at Fort Riley and Fort Leavenworth, Kansas, and at Fort Wayne, Indiana, from July 1865 to October 1873, and then at the U.S. Military Academy at West Point from October 1873 to September 1878. Col Irwin studied in Europe from September 1878 to September 1879, and then served as a surgeon at Fort Meade, Dakota, from October 1879 to June 1880. He next served in the office of the Medical Director for the Department of Dakota from June to August 1880, followed by service as attending surgeon at the Headquarters of the Department of the Missouri in Chicago, Illinois, from August 1880 to October 1882. Col Irwin served as Medical Director of the Department of Arizona from October 1882 to 1885, and he was then put in charge of the Medical Purveying Depot in New York from 1885 to 1886. He was in charge of the Medical Purveying Depot in San Franciso from 1886 to 1890, and he then served as Medical Director of the Department of Columbia at Vancouver Barracks from 1890 to 1891. His final assignment was as Medical Director of the Department of the Missouri at Chicago from 1891 until his retirement from the Army on June 28, 1894. At the time of his retirement, he was the second ranking officer in the Army Medical Corps. On April 23, 1904, he was promoted to the rank of Brigadier General on the Retired List for his service in the U.S. Civil War. BG Irwin was awarded the Medal of Honor for his heroism in 1861 during a ceremony just before his retirement from the Army, on January 24, 1894. He died on December 15, 1917, and was buried at the West Point Cemetery at the U.S. Military Academy in New York. His son, George LeRoy Irwin, graduated from West Point in 1889 and served in World War I, and died on active duty as a Major General in 1931. His grandson, Stafford LeRoy Irwin, graduated from West Point in 1915 and served in World War II, retiring as a Lieutenant General in 1952.

His Medal of Honor Citation reads:

Voluntarily took command of troops and attacked and defeated hostile Indians he met on the way. Surg. Irwin volunteered to go to the rescue of 2d Lt. George N. Bascom, 7th Infantry, who with 60 men was trapped by Chiricahua Apaches under Cochise. Irwin and 14 men, not having horses, began the 100-mile march riding mules. After fighting and capturing Indians, recovering stolen horses and cattle, he reached Bascom's column and helped break his siege.