Bernadette,
Would you tell us what age was on the death record? It's quite possible that age is incorrect.
Theresa herself obviously did not provide her age to the death registrar. It was provided by a third party who may not have known her exact age, may have been grieving, may have been stressed, may have been mistaken, etc. Or the registrar could have copied the age wrong. This goes for other info on death records. They are not as reliable as records where the person themselves provided the info.
John Johnson and Margaret McCormack were the names of Theresa's parents; she provided that info when she married in 1905. A couple by those names had a daughter, Bridget Theresa, in 1875. This may have been your ancestor. Or perhaps they had another daughter not shown on rootsireland.
People in Ireland were not as aware of age as we are. They weren't constantly being asked for ID. They didn't spend much time celebrating birthdays. Age wasn't on their radar like it is on ours.
I hope this helps.