Unemployed father-of-three Douglas Ward (36), of Loughantarve, Knockbridge, Co Louth, pleaded guilty to the manslaughter of Niall Dorr at the Central Criminal Court on October 30.
The attack took place on October 13, 2010, on Castle Road, in the centre of Dundalk.
In court, Niall's father David read an emotional victim impact report on behalf his family and said the ultimate result of Niall's horrific injuries was that he died a lonely death on the streets of his beloved home town.
"When the nurse handed him to me, this 6lb 9oz bundle of joy actually had me in tears. I didn't cry again for 18 years and 48 weeks, which is exactly how long we had him for.
"One of the toughest things myself and Shane (Niall's brother) will ever had have to do was to carry his coffin out of the house, and for his mam to walk behind us was equally tough on her. From now on we have must be ready to accept that news of other tragic deaths will bring us back to what happened to Niall," said David Dorr.
"This was cruel and heartless and I just hope when the accused looks at his children he will think of the young man he killed." He said even in his death, Niall showed his consideration for others as he saved the lives of five people by donating his organs.
In the Central Criminal Court Detective Insp Pat Marry told Brendan Grehan, prosecuting, a number of 999 calls were made at around 9pm stating a young man was lying unconscious on the footpath in Castle Road, partially dressed and underneath a parked car.
The teen, who had represented Ireland at the World Champion Kick Boxing, was identified as Niall Dorr and was rushed to Our Lady of Lourdes Hospital, Drogheda, where his condition deteriorated rapidly.
He was transferred to Beaumont Hospital, Dublin, where it was discovered he had irrecoverable brain damage.
The court heard Ward, who had been drinking all day, was with four men and two women who were trying to gain access to a house party at Castle Road when Niall Dorr and his two friends were walking by making their way home.
"It was a chance encounter between the groups," said Det Marry. "During the fracas, Mr Dorr became isolated with Mr Ward and he was assaulted in a very violent fashion," said Det Marry.
"Mr Dorr was trying to keep the peace," he added.
A witness told gardai she saw Ward "bouncing" Niall Dorr's head off the road up to 10 times and kicking him in the head a number of times. She said Mr Dorr was "like a rag doll and was totally unresponsive".
Another witness gave a graphic account to gardai saying it was a "continuous beating with vicious kicking and punching".
After the incident, Ward and another man got a taxi to an off-licence where they purchased more alcohol and returned to his home.
Ward will be sentenced on Friday.