David Kenneway, 27, was ordered by Belfast Court to spend at least 13 years behind bars
Mr Justice Stephen told the self-confessed killer that although he was dangerous, the risk he posed to the public would be a matter for the parole commissioners in 13 years time.
Kenneway, from Dundalk but with an address at Kenard Villas in Newry, had pleaded guilty to the murder of Paul Owens, 18, on 26 February 26 2011.
Mr Owens was fatally stabbed in the chest during a fight.
Kenneway’s former girlfriend, Lynsey Cahoon, 21, from Rodney Drive in Belfast, was handed a three year jail term but she walked free from the court after Mr Justice Stephens suspended that term for three years, telling her that but for the “wholly exceptional” features to her case, she would have gone to jail.
The judge also told Cahoon he accepted she had given police a misleading description, tried to clean some of Kenneway’s clothing as well as the murder knife, out of a “misplaced sense of loyalty” but that since then, she had fully co-operated with police and had expressed genuine and profound regret.
Mr Justice Stephens told Kenneway, who has 68 previous convictions including offences of possessing weapons, robbery and road traffic offences, that he accepted the killer had not instigated the fatal altercation.
The judge added that although Kenneway had also expressed remorse for killing the teenager and the devastation the murder had caused, he had “reservations about the long term duration” of that regret.
A previous court heard how he had been drinking with friends in the Donard car park in the town when one of his group traded insults with Kenneway before the killer was subjected to a “vicious and sustained” beating including being kicked “square in the face”.
Prosecution witness Mervyn Kelly helped him up and put him into a taxi to get him away from the scene but in what was described as a “cruel twist of fate” the taxi was already booked so he got out and was recovering on a wall when Paul Owens approached him again.
When Mr Kelly stood in front of him with his arms up and palms out, he and Mr Owens began to fight and it was at this time that Kenneway produced the knife and stabbed him three times, to the face, arm and chest.
It was the chest wound that caused his death.
Kenneway fled the scene back to the flat he shared with Cahoon on Central Promenade, later leaving to dispose of certain items.
Cahoon meanwhile gave police a fake description and when she went back to the flat, washed Kernneway’s trainers, jacket and boxer shorts as well as the knife but as the court heard, there was still blood all over the kitchen when police raided it at 5am.