Richard Dekker has been found guilty of the murder of Daniel McAnaspie by unanimous jury verdict.
The 30-year-old from the Blanchardstown area of Dublin will be sentenced on 3 April.
Daniel McAnaspie, 17, had been in HSE care in Dublin before his death in 2010.
Dekker from Dublin 15, pleaded not guilty to the murder at Tolka Valley Park, Blanchardstown, on 26 February 2010.
Both of Daniel's parents had died and he was living in Donabate when he was allowed out on the afternoon of 25 February 2010.
He had a mobile phone and was supposed to be picked up later, but his carers were unable to contact him and reported him missing to gardaí.
Daniel had been out socialising in Blanchardstown that evening and met a number of people, including Dekker, and went drinking.
Two of his friends tried to persuade him to leave with them at around 4am the following morning but he insisted on staying and that was the last time they saw him alive.
Two months later his badly decomposed remains were found in a deep drain on a farmer's land in Rathfeigh in Co Meath. He had been stabbed to death.
Dekker was arrested and admitted he was with Daniel before he was killed.
Daniel had been persuaded to go to an isolated area along the River Tolka in the early hours of 26 February.
Dekker was there when he claimed another man brought half a garden shears with him.
Dekker described in detail to gardaí how that man killed Daniel with the shears. He told them the teenager begged for his life and he also pleaded with the man to stop but that he then got scared that he would use the blade on him.
Dekker also showed gardaí where the killing happened but insisted he did not know it was going to take place.
There were shouts of "Justice for Daniel" when Dekker was taken into custody after the verdict was read out.
Dekker's conviction came after a week-long trial and more than eight hours of deliberation by the jury at the Central Criminal Court.
When the foreman confirmed that the verdict was unanimous, at least one of Daniel's family could be heard whispering "well done, well done".
When Justice Patrick McCarthy said he would remand Dekker in custody until a sentencing hearing on 3 April, they cheered and clapped.
One shouted: "Well done judge, well done jury" while others shouted "Justice for Daniel", a slogan that has been emblazoned on t-shirts worn by the victim's family during the trial.
Prosecuting counsel Brendan Grehan SC said Daniel's family will prepare a statement for the court which will be read out at the sentencing hearing.
Having been convicted of murder, Dekker faces a mandatory life sentence.
Trevor Noone, 28, also from Blanchardstown, pleaded guilty to manslaughter last month for his role in the boy's death. He will also be sentenced in April.