Cronin was born in Dublin in 1920 but spent his childhood years in Ballinskelligs, in the County Kerry Gaeltacht. During the Second World War, Cronin was an officer in the Southern Command. He later emigrated to New York, where he found work as a journalist. In America, he became involved with Clan na Gael and later joined the Irish Republican Army. In 1955 he returned from the United States and began work as a subeditor in the Evening Press. Jailed for his activities, he left the IRA in 1962 after his release from prison.