Gardai have named two of the victims of the Sligo fire as 43-year-old Eugene Coyle and Kathleen Divers, 36.
The victims were found dead in bedrooms upstairs.
A man and two sisters died when fire swept through a house in St Bridget's Place, Sligo.
Three others escaped when fire broke out in the early hours at the house.
Another woman, yet to be formally identified, also died in the blaze. Fire crews were on the scene within minutes of the fire being reported to the police at 2.30am.
The survivors were taken to Sligo General Hospital but were not seriously injured and were later released.
Gardai and fire brigade officials are still trying to establish the cause of the fire.