Hello Jon_ni,
Thank you for the links and the tip about the paper copy. Catherine's brother Thomas is also in the 1871 census, maybe also from Wexford, and I'm not making any head way tracking them down, so maybe there's a chance he's on the marriage certificate, and/or James' brother Patrick.
I followed up the Griffiths Valuation and located a cluster of the Connor family together. Its a brilliant tool.
Thanks for your help, rejen