Hi again Aghadowey,
First, well spotted for Catherine 1874, thank you! It is complicated when some are registered as Antrim, others as Belfast.
I now have 8 children:-
1874 Catherine (your discovery)
1876 Charles 1876
1877 Elizabeth
a potential gap for a couple of births
1880 Annie
a potential gap for another birth
1882 Catherine
a potential gap for another birth
1884 John
a potential gap for a couple of births
1887 Sarah
a potential gap for a couple of births
1891 Stanislaus
You have found two more?
In the 1911 census Patrick only referred to his children with Sarah Lyttle (5 born, 3 living).
Lennon Wylie show a Catherine O'Neill (widow) at 57 New Dock Street in 1880 & 1890; the street did not seem to exist in 1868.
Patrick O'Neill's second marriage (to Sarah Lyttle) 12/8/1900 records his father as Charles O'Neill (deceased). We have not yet found a parental Charles+Catherine connection. There seems to be a Bernard somewhere in the mix. However you very rightly suggest Catherine as the potential mother-in-law to Elizabeth Ellen.
We also find no trace of Patrick & Elizabeth's marriage in Ireland. Family tales say Patrick saw her with her family in a carriage on a dockside, then followed on foot for days until he could meet her (it was never clear where that may have happened). There is also a 'yarn' about her dressing as a man to board ship and run away with him. That all sounds far too dramatic to be true; who knows? He has never been recorded as a sailor.
So far, with only rudimentary skills, I have looked, but not found their marriage in the US. Norfolk was chaos at that time due to the epidemic and records were not consistently maintained (they're also very difficult to access and to read). In all events, the Kearneys had properties everywhere, and a marriage may have occurred well outside Norfolk. Wise people tried to flee the yellow-fever epidemic, many were repulsed elsewhere, but the very wealthy probably got by. They still do!
Many, many thanks for your time, thought, and suggestions.