At the age of 18, in 1846 he took an appointment to the Revenue Police, enforcing the laws and searching for illicit stills and distilling activities in Creeslough, County Donegal. He also joined a guerrilla group called the Ribbonmen.

On August 30, 1849, he emigrated from Sligo Bay to the U.S. and settled in New York City where he found work as a clerk in the tavern, Hibernian House, at 42 Prince Street in Manhattan owned by John Heaney, whose niece, Elizabeth, he married in 1854.

He enlisted as a Private in the 69th New York Militia. By 1859 he was appointed colonel of the regiment. The regiment was a state militia unit at that time composed of citizens, not soldiers, and was involved in the maintenance of public order. On October 11, 1860, Colonel Corcoran refused to march the regiment on parade for the 19-year-old Prince of Wales, who was visiting New York City at the time, protesting the British imposition of the Great Famine of Ireland. Corcoran was removed from command and a court martial was pending over that matter when the Civil War began.

While Corcoran was imprisoned the U.S. had made threats to execute captured Confederate privateers. Corcoran and several other Union prisoners were selected by lot for execution if the U.S. carried out its threats against the privateers. This event was known as the Enchantress Affair, but no executions were ever carried out by either side. Corcoran was then offered a parole under the conditions that he not take up arms against the Confederacy. Intending to resume his place in the Union army upon his release he refused the offer of parole. He was appointed Brigadier General of volunteers in July and exchanged in August 1862. His role in the Enchantress Affair and his refusal for parole gained him some attention and upon his release he was invited to dinner with President Abraham Lincoln.

In April, 1863 Corcoran was involved in an incident that ended with Corcoran shooting and killing Edgar A. Kimball, commander of the 9th New York Volunteer Infantry Regiment. Corcoran attempted to pass through the 9th New York’s area without giving the required password after receiving the challenge from a sentry. When Kimball intervened on the side of the sentry, Corcoran shot Kimball. Corcoran was not charged with a crime or reprimanded, and continued to serve.

Placed in command of the 1st Division, VII Corps he was engaged in the Battle of Deserted House and took part in the siege of Suffolk. In late 1863 he was placed in command of a division in the XXII Corps and returned to serve in the Washington defenses. While riding alone in Fairfax, Virginia he was thrown from a runaway horse and suffered a fractured skull. He died at the W.P. Gunnell House on December 22, 1863 at the age of 36.

The Mayor of New York City, Michael Bloomberg unveiled Ireland’s national monument to the Fighting 69th in Ballymote on August 22, 2006. The monument was sculpted by Philip Flanagan. The inscription around the top of the monument reads “Michael Corcoran 1827–1863” Around the base is inscribed “New York Ballymote Creeslough Bull Run”. Underneath the monument is a piece of steel from the World Trade Center, donated by the family of Michael Lynch, who died in the tower on September 11, 2001. Lynch’s family are from County Sligo.