The couple have been named locally as Michael Greaney, 53, and his wife, Valerie, 49.
They were discovered at a house at O'Neill's Place, Cobh, at about 15:30 local time on Sunday.
Their eldest daughter is in hospital, the other is said to be traumatised.
Police were alerted by neighbours who contacted them after the couple's 16-year-old daughter was told by her mother to raise the alarm.
They found the other daughter, 21-year-old Michelle, lying on the road outside after a knife attack.
She was treated at the scene before being taken to hospital where she remains in a serious but stable condition.
The couple's youngest daughter was not hurt, but is said to be severely traumatised.
Police say they are not looking for anyone else in connection with an incident in which a married couple were found dead in their home in County Cork in the Republic of Ireland.