Students Neil McFerran and Aaron Davidson, who were both 18 and from Newtownabbey, died in August 2010 after they were overcome with the poisonous gas which leaked from a gas system that had not been properly installed.
George Brown - the former owner of a gas store in Coleraine - admitted causing the manslaughter of the two friends on the grounds of gross negligence.
And while he said he didn't install the boiler and flue, he accepted the supply and installation of the heating system was his responsibility.
Brown, a 52-year old father-of-two from Ballygawley Road in Aghadowey, also admitted a string of other charges including health and safety breaches relating to work undertaken at the apartment in Castlerock, as well as defective workmanship carried out by both him and his employees in and around the greater Coleraine area.
Crown prosecutor Frank O'Donoghoe told Coleraine Crown Court, sitting in Belfast, that a new boiler flue was fitted in the apartment in March 2010, and the owner - who was Aaron Davidson's aunt - was invoiced for the supply and installation by the Brown's shop.
When an investigation into the installation was launched in the aftermath of the deaths of the two young men, it emerged there had been a failure to properly secure a join in the flue system, which allowed carbon monoxide to leak into the apartment.
A section of the flue was completely separated, while the collar between both sections of the flue was not secured with screws to hold it in position.
Mr O'Donoghoe said that in June 2010, the owner stayed in the apartment at Tunnel Brae and that during her stay she switched on the gas boiler.
She started to feel sick and after drifting in and out of consciousness, she vomited twice, he told the judge.
She was subsequently treated for vertigo, but Mr O'Donoghoe said that with hindsight the owner believes "the symptoms she experienced...were symptoms of carbon monoxide poisoning as the boiler engaged".
Turning to the events of the fatal weekend in August 2010, Mr O'Donoghoe revealed that Neil, Aaron and another friend, Matthew Gaw, stayed in the apartment and were due to return home on 3 August.
Matthew, who sustained near fatal injuries as a result of carbon monoxide poisoning, described waking up on the bathroom floor before stumbling around the apartment and trying to wake Aaron, who was lying in the living room.
This was characterised by shoddy, dangerous workmanship and this man presided over it.
Mr Justice Weir
Mr O'Donoghoe said the families of Neil and Aaron became anxious after they were unable to reach their sons on the phone, prompting them to drive to the apartment.
When they arrived, the two fathers forced their way in to the apartment, where they found Aaron and Neil lying on the floor and Matthew sitting in a chair.
Despite the efforts of the emergency services, both Neil and Aaron were pronounced dead at the scene.
When an investigation was launched, it emerged that as well as the boiler and flue not being installed correctly causing it to leak carbon monoxide, the boiler had not been properly converted from natural gas to liquefied petroleum gas.
This in turn led to the boiler producing higher levels of the poisonous gas.
Mr O'Donoghoe said that while it was not possible to determine who had carried out the installation which included an unqualified apprentice employed by Brown, the work fell "well below" the standard expected of a competent and registered gas installer.
Defence barrister Eilish McDermott said that as a result of the fatalities, Brown "closed his business on the day that he heard about the deaths of the two young men, and he hadn't opened it since".
Saying her client was remorseful for the deaths of Neil and Aaron, the defence barrister spoke of the "unspeakable" suffering of the families of the two students, adding the deaths were "a complete travesty".
Ms McDermott added that while Brown said he was not the person responsible for carrying out the installation, his plea to manslaughter was accepted "on the basis he should not have sent out someone who was an unqualified operative to do this work".
Saying that Brown and his employees carried out "highly sub-standard work and dangerous gas installations" not just at Tunnel Brae but at other properties in the area, Mr Justice Weir said it was fortunate "that we only dealing with one incident of fatality and not a whole lot more".
He told the court: "This is an unmitigated tragedy for everyone and could so easily have been avoided with just a little bit of care."
Mr Justice Weir will sentence Brown next Thursday.