During the Second World War, Cronin was an officer in the Southern Command. He later emigrated to New York City, where he found work as a journalist. In America, he became involved with Clan na Gael and later joined the Irish Republican Army.
He was soon put in charge of training in the IRA. He outlined his ideas in a booklet, Notes on Guerrilla Warfare. He became the chief strategist for Operation Harvest, a campaign which saw the carrying out a range of military operations from direct attacks on security installations to disruptive actions against infrastructure. He was arrested and imprisoned several times over the course of this campaign (1956–1962).
Jailed for his activities, he left the IRA in 1962 after his release from prison.
He was the author of a dozen books and pamphlets, including a biography of republican Frank Ryan, Washington's Irish Policy 1916-1986: Independence, Partition, Neutrality, an authoritative account of Irish-US relations; Our Own Red Blood about the 1916 Easter Rising; and a number of works on guerrilla strategy, including an early Sinn Féin pamphlet Resistance under the pseudonym of J. McGarrity.
After several years of illness, Cronin died in Washington on 9 March 2011. He is survived by his second wife, Reva Rubenstein Cronin.
- Wolfe Tone. Dublin Directory: Wolfe Tone Bi-Centenary, 1963.
- Jemmy Hope: A Man of the People. Scéim na gCeardchumann, 1964.
- The Story of Kevin Barry, with a foreword by Commander-General Tom Barry. The National Publications Committee, 1965.
Reprinted by C.F.N. in 1983.
- Our Own Red Blood :The Story of The 1916 Rising. Wolfe Tone Society, 1966 Reprinted 1976 and 2006.
- The Rights of Man in Ireland. Wolfe Tone Society, 1970.
- The Revolutionaries. Republican Publications, 1971.
- Ireland Since The Treaty : fifty years after. Irish Freedom Press, 1971.
- The McGarrity Papers: Revelations of the Irish Revolutionary Movement in Ireland and America, 1900-1940 Anvil Books, 1972.
- Freedom the Wolfe Tone Way Anvil Books, 1973.
- Tone's Republic : The Case Against Sectarianism Wolfe Tone Society, 1975.
- Marx and the Irish Question. Repsol, 1977.
- Frank Ryan : The Search for the Republic. Repsol, 1980.
- Irish Nationalism : a history of its roots and ideology. Academy Press, 1980, US edition,
- Washington's Irish policy 1916-1986 : independence, partition, neutrality. Anvil Books, 1987.
- For Whom the Hangman's Rope was Spun : Wolfe Tone and the United Irishmen. Repsol, 1991.
- Brian Hanley and Scott Millar, The Lost Revolution: The Story of the Official IRA and the Workers Party, pp. 12-13, ISBN 1-84488-120-2
- Patrick Smyth, "Veteran republican and first 'Irish Times' Washington correspondent dies aged 91", Irish Times, 10 March 2011.
|Editor of the United Irishman
Seán Ó Brádaigh