Kammala jumla mai zuwa: An haifi O'Shannon a Marino, Dublin, Ireland, a ranar 23 ga Agusta 1928, ɗan Cathal O'Shannon (Sr.), ɗan gurguzu kuma ɗan Jamhuriyar Irish. Ya sami ilimin sa na yau da kullun a Makarantar Colaiste Mhuire, a cikin Parnell Square, Dublin. Duk da siyasar mahaifinsa, a matsayin O'Shannon mai shekaru 16 ya ba da kansa don yin aikin yaƙi tare da Royal Air Force a Belfast a 1945 a lokacin Yaƙin Duniya na II, ta amfani da takardar shaidar haihuwa ta karya don ɓoye kasancewarsa ƙarami don shiga cikin Sojojin Burtaniya. Bayan horar da ma'aikatan jirgin sama an tura shi zuwa Gabas ta Tsakiya, a matsayin mai harbi na baya a cikin jirgin Avro Lancaster mai jefa bam don shiga cikin Kamfen na Burma, amma yakin ya ƙare tare da
faduwar Daular Japan kafin a bukaci ya tashi a yakin basasa.