The four women found dead in their home in Leixlip, Co Kildare, last week were cremated at the weekend.
Preliminary post-mortem results show that the women died from carbon monoxide and carbon dioxide poisoning, according to a spokeswoman for the Garda Press Office.
Gardai have said they are convinced the women died from accidental poisoning caused by the build-up of fumes from the central heating in their home at Rinawade Grove.
Further toxicology tests are to be carried out to establish the exact cause of the women's deaths, the Garda spokeswoman said last night.
Mrs Frances Mulrooney (83) and her three nieces, Josephine (46) and twins Catherine and Ruth (51), were found dead by their landlord last Wednesday.
The doors of the house had been barricaded with furniture and a fridge and they had blocked the ventilation.
The women had blocked up the doors and windows of their former home in Sandymount, Dublin, according to neighbours, who said the four were anxious to prevent draughts.
They were described as reclusive by neighbours in Leixlip, who rarely saw them.
The verdict on the women's deaths will be given by the Kildare County Coroner, Mr Denis A. Cusack.
The women were cremated in Dublin at the weekend after a funeral Mass at St Mary's Priory in Tallaght attended by their surviving sisters and friends.
A Fianna Fail TD, Mr Conor Lenihan, attended the Mass and sympathised with a relative of the dead women.
A sister of three of the dead women sang The Isle of Innisfree at the end of the service.