O'Shannon was born in Marino, Dublin, Ireland, on 23 August 1928, the son of Cathal O'Shannon (Sr.), a Socialist and Irish Republican. He received his formal education at Colaiste Mhuire School, in Parnell Square, Dublin. Despite his father's politics, as a 16-year-old O'Shannon volunteered for war time service with the Royal Air Force in Belfast in 1945 during World War II, utilizing a forged birth certificate to disguise being underage for enlistment with the British Armed Forces. After air crew training he was posted to the Far East, as a rear gunner in an Avro Lancaster bomber to take part in the Burma Campaign, but the war ended with the downfall of the Japanese Empire before he was required to fly combat sorties.