.
It's thought that a cigarette or an electrical appliance sparked the fire at the house, above her father's butchers shop in Castlewellan, Co. Down, just after 4am.
However firefighters were unable to officially determine why the blaze started, but her father Bartley Finnegan, who was not home that night and had been due to go on a skiing holiday the following day, said there was an electric heater in the house.
Forensic scientist Julian Halligan was unable to be definitive about the cause but suggested it may have been from an electrical heater plugged into a hallway socket or careless disposal of cigarettes.
He said the most extensive damage was in the doorway area of one of the bedrooms, where the fire had burned through the floor into the ceiling of a butcher's shop below.
A box of matches was found in the bathroom. But Ms Keenan said her friend left her cigarettes in her car.
The inquest was told that there was no smoke alarm at the property and the blaze was only discovered by a roads service worker out gritting who called emergency services.
Firefighters found the teen's body in the bedroom of the house.
The coroner ruled Ellen died from smoke inhalation during the house fire.
Coroner Suzanne Anderson said: "Sadly this case does highlight the very grave risks involved when alcohol is involved, in particular heavy consumption of alcohol.
"I hope that lessons can be learned."
Around 1,000 mourners lined the street in Castlewellan in January to pay tribute to the popular grammar school girl.
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