A member of the public alerted authorities after finding a vehicle in flames in a wooded area near Doneraile town, about 30 miles north of Cork city, at around 4.30pm on Thursday.
Police and fire crews responded and found the woman inside the car. She was pronounced dead at the scene.
A man was later rescued from a nearby river.
It is believed the deceased woman, who was aged in her 70s, and the man, in his 60s, were known to one another.
Police say the woman lived locally while the man resides in Co Limerick.
The man was rushed to Cork University Hospital for treatment.
His condition was said to be serious but improved overnight. He is set to interviewed by police when his condition further improves.
Investigators are working to establish the circumstances surrounding the woman's death.
The burnt-out car and a second vehicle found close by have been designated as crime scenes.
Assistant state pathologist Dr Margot Bolster will carry out a post-mortem examination later on Friday.
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