I cannot be 100% sure of Ellen. I base it on the fact that there is an Ellen born ( Name Ellen Sargint Birth date: 08 Oct 1870 Birthplace: Cappoquin, Waterford (Sub District) Ref. : 817 Parent : Michael Sargint & Anne Tracy (Familysearch records) )
The death for Ellen in Waterford death registers :
Date of Death: 16 Oct 1870
Place of Death: Mill St
Name: Sargint, Ellen, Sex: F, Age at Death: 1 Week, Rank: Carpenters Child, Condition: Child
Cause of Death: Convulsions
Informant: Tracy, Mary
Qualification: Present at Death, Residence: Castle St
So the Ellen born with named parents Michael and Anne fits the name/age( 1 week) of the Ellen death in 1870.
I averaged out the age variance for the girls in the 1901 census but yes you are right.
It seems a very big age difference , even for census records? I fully expected to find registrations But I can't find any civil records for the 1901 girls.
I cannot work this scenario at all. Even by the rest of my Irish family , this one is puzzling.