Hi,
If none of the census give a place of birth more specific than Co Cork, and you haven't found their marriage in England, the possibilities are limited with such common names. Co Cork valuation was taken between 1851 and 1853 so they may be on it and may not. So may many other John O'Connell households so how would you know if you had the right one? If there are only a limited number, you could try going to parish records for marriages and hopinf he named one of his sons after his father...
Kate