The 1916 Easter Rising was a watershed in O'Donoghue's life. In December 1916, Florence joined the Cork branch of the Irish Volunteers. In early 1917, he was elected unanimously 1st Lieutenant of the Cyclist Company and as result devoted all his spare time to Volunteer work. He began writing weekly for two years for the "Irish World" newspaper. By May 1917, he was sworn into the Irish Republican Brotherhood and in October, Tomás Mac Curtain appointed O'Donoghue as communications of the Cork Brigade. He replaced Pat Higgins as Brigade Adjutant in February 1917. O'Donoghue was a key organiser in the sensational jail-break of Captain Donnchadh Mac Niallghuis on Armistice Day 1918 and took personal responsibility for his protection. Michael Collins was the last officer from Volunteers General Headquarters to visit Cork shortly after Christmas 1919, until the truce in 1921.