Civil War Union Army Officer. Born in Borrisokane, County Tipperary, Ireland, he came to the United States in 1843, and became a builder in Philadelphia, Pennsylvania. Soon after the outbreak of the Civil War he was commissioned as Lieutenant Colonel of the 24th Pennsylvania (Three-Month) Volunteer Infantry on May 1, 1861. The unit, commanded by future Union General Joshua T. Owen, served in the Shenandoah Valley during the First Bull Run Campaign, and saw no significant action. Mustered out on August 10, 1861, he was authorized in 1862 to recruit a new regiment. Drawing largely from the Irish population in Philadelphia, that unit was mustered into service as the 116th Pennsylvania Volunteer Infantry, and Dennis Heenan was commissioned its Colonel and commander on September 1, 1862. After some initial garrison duty in Virginia, his regiment was assigned to fill the depleted ranks of the now-Famous Irish Brigade of the Army of the Potomac, which had been decimated in action at the Battles of Second Bull Run and Antietam. In the December 1862 Battle of Fredericksburg, Virginia, Colonel Heenan led his men through the town during his division's part the Union Army's ultimately futile assaults on Confederate positions at Marye's Heights. Just before his men reached a canal that ran along the open fields outside of the town, his left hand and arm was severely wounded by an artillery burst. He was brought off the field, handing command over to Lieutenant Colonel St. Clair Augustin Mulholland, who was also his son-in-law. After he wounds had been dressed, he returned to the carnage of the battlefield to assist his men, and brought off the regimental colors when the survivors retreated. The charge caused great casualties amongst his regiment, and when it was consolidated in January 1863, it was made into a battalion of four companies. The consolidation made Colonel Heenan a supernumery officer with a rank unequal to the size of his command, so he was honorably discharged on January 26, 1863. He returned to Philadelphia, and eventually became a public works contractor before dying of injuries from a carriage accident in July 1872.