A member of the IRB since 1901, Denis O’Callaghan was made centre of his circle in 1908. He was also a member of Na Fianna Éireann and became Captain of A Company in the 1st Dublin Battalion of the Irish Volunteers shortly after their establishment. He fought in the Four Courts area during the Easter Rising, leading attacks on the Broadstone Railway Station and the Linenhall Barracks. He was subsequently court-martialled and sentenced to death, but this was commuted to ten years penal servitude and he was interned in Portland and Lewes prisons until June 1917. O’Callaghan undertook organisational work for the Volunteers and electoral work for Sinn Féin following his release and later served in the Volunteers’ intelligence department.
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