During the Irish War of Independence he was one of those who worked very closely with Michael Collins and was known as one of his "best intelligence men.
[2] On the same day as the Bloody Sunday (1920) massacre he was one of the gunman involved in the shooting of 18 suspected members of the Cairo gang.
[3] According to Irish Historian Tim Pat Coogan, Cullen and two other intelligencers broke the Dublin Castle spy system.
[4] He died from asphyxia due to accidental drowning (according to his death certificate), on 20 June 1926[6] and was buried in Rathnew cemetery.
Up until the mid 1960s, veterans from Dublin and Wicklow held an annual commemoration in his honor.