The infant girl was in her bedroom in a house in the village of Clashmore when she was attacked early on Monday morning, according to local media.
She sustained serious head injuries during the attack, RTE reported.
She was taken to Cork University Hospital where she was pronounced dead shortly after 3.15am, Irish police said.
An investigation is under way into the incident and a post-mortem will be carried out today, the police added.
In a statement, they said: “Gardai are investigating all the circumstances after a female infant, aged 3 months, was fatally injured by a dog at a residence in Clashmore, Co Waterford, in the early hours of this morning, 7 June 2021.
“Gardai and paramedics treated the infant at the scene until she was taken to Cork University Hospital. She was pronounced deceased a short time later.
“An Garda Siochana are appealing to the media and public to respect the privacy of the family at this time.”
Friar Milo Guiry of Ardmore described the incident as a “terrible tragedy”.
“We will keep the family in our prayers. Every support will be offered to them. It is terrible, terrible news,” he said.