Where have you been looking for death/birth records?
First you need to look at what was available:
https://familysearch.org/wiki/en/How_to_Find_New_York_Birth_Records#Births_from_1851_thru_1880After the failure of the 1847 law, no births were recorded by the State of New York between 1850 and 1880 though some cities (including Albany, Buffalo, Yonkers) started recording births in the 1870s.
So we do not expect to find birth records for Ellen and Bridget, although there might be other records such as church records. Do you know what church the family might have attended?
Similarly, if they died before 1880:
https://familysearch.org/wiki/en/How_to_Find_New_York_Death_Records#Deaths_from_1851_thru_1879After the failure of the 1847 law, no deaths were recorded by the State of New York between 1850 and May 1880 though some cities (including Albany, Buffalo, Yonkers) started recording deaths in the 1870s.
Again, church records, cemetery records, probate records, etc, could stand in.
I think possibly Cornelia died and Hugh remarried.
In 1875 NY census, a Hugh McGaffin is in the Poughkeepsie area with a Sophia as wife and no kids:
https://familysearch.org/ark:/61903/1:1:VNVV-2GHIn 1880 he is with her and three kids in NJ:
https://familysearch.org/ark:/61903/1:1:MNZB-6VXstill around there in 1900:
https://familysearch.org/ark:/61903/1:1:M9JS-2P3Note: occupation shoemaker
and 1910:
https://familysearch.org/ark:/61903/1:1:MKTT-MP7Years married listings in 1900 and 1910 suggest they married around 1874-1876, with other records such as census of 1875 and the first child being born 1875 suggesting the 1874 date is more likely.
In 1910 Sophia has "m1" and Hugh "m2", i.e. she was married once and he, twice.
Interestingly in 1900 Hugh seems to have first entered "40" for number of years married, which might fit if he listed the number of years since his marriage to Cornelia?
So you are looking for a burial/cemetery/newspaper etc. record for Cornelia's death between 1870 and 1875. No single "death record" as such as they weren't kept at that time. I would suggest looking for family members - for example Cornelia's relatives - to see if the girls were placed with them, perhaps the surname has been misspelt.