Citation

Under fire of the enemy, successfully destroyed a valuable train that had been abandoned and prevented it from falling into the hands of the enemy.

http://www.cmohs.org/recipient-detail/904/mcmahon-martin-t.php

http://en.wikipedia.org/wiki/Martin_T._McMahon

Martin Thomas McMahon (March 21, 1838 – April 21, 1906) was an American jurist and a Union Army general during the American Civil War. He was awarded the United States military's highest decoration, the Medal of Honor, for his actions at the Battle of White Oak Swamp. After the war, he held various legal and judicial positions in the state of New York. He briefly served as the Minister Resident to Paraguay and was a New York State Senator for four years.

McMahon was born in Laprairie County, Quebec, Canada,[1] to a family of recent immigrants from Waterford, Ireland. The family moved to the United States when McMahon was an infant and settled in New York. He graduated from St. John's College, Fordham, in 1855 and then studied law in Buffalo,[2] receiving his Master's degree in 1857.[1] After his schooling, he traveled west and worked as a special agent for the post office on the Pacific coast. He was admitted to the Sacramento, California, bar in 1861

McMahon's two older brothers were also officers in the war, both with the 164th New York Volunteer Infantry Regiment. John Eugene McMahon (1834–1863) commanded the 164th before being injured; he later died of these injuries. Middle brother James Power McMahon (1836–1864) took over the regiment and led it until his death at the Battle of Cold Harbor.

His tenure in Paraguay: http://www.irlandeses.org/1003huner.htm