[1] Cronin was born in Dublin but spent his childhood years in Ballinskelligs, in the County Kerry Gaeltacht.

[2] During the Second World War, Cronin was an officer in the Southern Command in the Irish Defence Forces.

In America, he became involved with Clan na Gael and later joined the Irish Republican Army.

[3] Cronin believed that a strong campaign of attacks on police barracks, military installations and government buildings would force the withdrawal of security forces from villages and small towns thereby making large areas of the north ungovernable.

[2] 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.