Born in Dublin, Harrington joined the Irish Republican Army (IRA), and fought in the Irish War of Independence. He opposed the Anglo-Irish Treaty and remained with the IRA through the Irish Civil War.
By 1941, Harrington was a member of the IRA's Army Council which organised a court-marshal of former Chief of Staff Stephen Hayes, although he did not personally take part in the trial. He became Chief of Staff of the IRA in November 1941 when Pearse Kelly was arrested, but he himself was arrested the following month. He was charged with failing to explain his possession of £50 and refusing to give his details to police, and was interned for two years. Despite this, he appears to have remained Chief of Staff until February 1942, when Seán McCool was selected as his replacement.
On release, Harrington relocated to Dublin, where he found work as the caretaker at the Court Laundry. This was a difficult role, as local levels of crime were high. In December, he was tied to his bed and beaten, taking three months to recover; he apprehended an armed robber in April 1946. He broke his links with the paramilitary movement and also left the Catholic church, joining a group of Quakers; this group attracted some opposition, and Harrington lost the hearing in one ear after being attacked by a group of young Catholic extremists.
- Muriel Seltman, What's Left? What's Right?, p.17
- T. J. Barrington, Discovering Kerry: Its History, Heritage & Topography, p.128
- Muriel Seltman, What's Left? What's Right?, p.24-25
- Richard English, Armed Struggle: The History of the IRA, p.56
- Edgar O'Ballance, Terror in Ireland: The Heritage of Hate, p.67
- "Irish news reel: Munster", Irish News, 14 March 1942
- Uinseann MacEoin, The IRA in the twilight years: 1923-1948, p.857
- "House-breaking in Dublin", Irish News, 1 April 1946