Origins of my great-great-grandmother Johanna Fahey and her family

Good afternoon my fellow family seekers, I'm researching the origins of my great-great-grandmother Johanna Fahey and keep hitting a roadblock. So far, I know she was born in July of 1855 to Patrick Fahey & Maria Crowley, and according to records was baptized  at Sts Peter & Paul Cathedral in Clonmel, Tipperary. At least some of her younger siblings were baptized there as well, before immigrating to Chicago in 1869. The only catch in all this is that her death certificate says she was from County Waterford, even though her family were all baptized in Sts Peter and Paul, Clonmel. and death certificates of her mother and other relatives also say Tipperary, which from my understanding is solely in County Tipperary and not Waterford. Why would that be, if anyone knows about this, I would love to solve this discrepancy, thank you so much!!