Paul Owens, 18, was stabbed to death on the Bryansford Road, in Newcastle, last February. He died later in hospital.
David Kennaway, 27, from Kenard Villas, was due to have gone on trial for the killing but changed his plea to guilty.
His former girlfriend, 21-year-old Lyndsey Cahoon, of Central Promenade, Newcastle, has been charged with  three counts of assisting an offender.
She allegedly gave a false description of the attacker and later was said to have washed Kennaway's clothes and a knife.
While no details were given to the court surrounding the stabbing, the prosecution handed in a set of agreed facts, which also outlined the mitigating and aggravating factors in the case, and the precise time when Kennaway armed himself.
The prosecuction lawyer said it was hoped that this "matrix" of the case would provide some assistance to the court when it came to impose Kennaway's tariff and in those circumstances he did not wish to open the document until then.
Mr Justice Stephens said he was "obliged" to imprison Kennaway for life.
A Newry man has been jailed for life for the murder of a County Down teenager last February.