Whakakiia te rerenga e whai ake nei: I whanau a O'Shannon ki Marino, Dublin, Ireland, i te 23 o Akuhata 1928, te tama a Cathal O'Shannon (Sr.), he Socialist me Irish Republican. I whiwhi ia i tana maatauranga i te Kura o Colaiste Mhuire, i Parnell Square, Dublin. Ahakoa nga kaupapa tōrangapū a tona papa, i te 16 o ona tau o O'Shannon i tuku mo te ratonga pakanga me te Royal Air Force i Belfast i te 1945 i te Pakanga Tuarua o te Ao, ma te whakamahi i tetahi tiwhikete whanau tinihanga hei huna i te mea he iti ake te pakeke mo te whakauru ki nga Rererangi o Ingarangi. Whai muri i te whakangungu a te kapa rererangi, i tukuna ia ki te Rawhiti ki te Rawhiti, hei kaipupuri pupuhi i muri i roto i te kaipupuri pupuhi Avro Lancaster ki te whai waahi ki te Whakatairanga o Burma, engari i mutu te pakanga me te
te hinganga o te Emepaea o Hapanihi i mua i te tono kia rere ia ki te whawhai.