The following may help but is unproven:
On the O'DONNELL side
The witnesses to Patrick and Catherine’s marriage were Nicholas COLLINS and Nora O’DONNELL
Possibility of 1901 census for Nora and her parentsIf the information above is correct, this is the same couple on 1911 censusshowing they had seven children of which only two were still living
implying Nora and Catherine if they were sisters, which is the presumption I am making here
Nicholas COLLINS was the name of an RIC Constable in Waterford in 1905
(from court session records)
He was born in Tipperary and enlisted in 1895
The 1911 census lists a family of Nicholas & Nora Collins and young son Jeremiah There is a marriage of Nicholas COLLINS and Nora O’DONNELL on April 30th 1907 in Lismore, Co. Waterford
Civil Reference LISMORE 1907 Q2 V4 P337
Nicholas was possibly born Apr 16 1877 to Jeremiah COLLINS and Mary LACY;
this would tie with the eldest child being named after his paternal grand father
Jeremiah Francis COLLINS was born on Aug 22 1908 KILMACTHOMAS Q4 V4 p526
Mary COLLINS was born on Jul 05 1911 KILMACTHOMAS Q3 V4 P533
There is a possible marriage in Cork for Jeremiah Francis COLLINS in 1942 CORK Q3 V5 P150
There are children born in Cork after that date
with a match to one of the four listed possible spouses on the record
with names that seem to be family names – however I cannot list here as they may still be living.
There is a death in the UK in Richmond upon Thames, Surrey, October 1990
DoB 22 August 1908 aged 82 of Jeremiah Francis COLLINS
Again if proven correct, Jeremiah and Mary would be your KELLY family’s first cousins
and some may be UK based rather than Ireland based
Just also to note that the marriage in 1905 of Patrick O'KELLY states he was from Ferns, Co. Wexford not Mitchelstown, Co. Cork as you have mentioned