There were roars of "justice for Daniel" as Richard Dekker was taken into custody after being found guilty of the murder of 17-year-old Daniel McAnaspie.
The teenager was in the care of the HSE before his death at Tolka Valley Park in north Dublin on February 26, 2010.
His decomposed body was found by a farmer in a seven-foot deep drainage ditch in Rathfeigh, Co Meath, 30km from where he was killed.
Dekker (30), from Blanchardstown, Dublin, was found guilty by a unanimous jury verdict of Daniel's murder.
Trevor Noone (28), also from Blanchardstown, pleaded guilty to manslaughter last month for his role in the boy's death.
Dekker's conviction came after more than eight hours of deliberation by the jury at the Central Criminal Court.
As Justice Patrick McCarthy announced he was remanding Dekker in custody until a sentencing hearing on April 3, Daniel's relatives cheered.
One shouted: "Well done, judge, well done, jury," while others shouted "justice for Daniel", a slogan that had been emblazoned on their T-shirts.
Dekker himself did not react, but sat with his eyes closed.
The conviction made legal history as it is the first time a person has been retried under the Criminal Justice Procedure Act 2010, which allows the State to retry a person even after they have been acquitted.
Dekker faced trial in 2013 but was acquitted by the trial judge on the grounds that there was insufficient evidence. The State went to the Supreme Court to overturn that decision.
Prosecuting counsel Brendan Grehan SC said Daniel's family would prepare a statement for the court which would be read out at sentencing. Having been convicted of murder, Dekker faces a mandatory life sentence.
Outside the court, Detective Superintendent Colm Fox, the senior investigating officer, said Daniel's murder was a "heinous act of violence against a child. Justice has been done".
On the day he died, Daniel's carers dropped him off in Finglas where he was going to meet some friends. His curfew was 9pm but he skipped it and went to Blanchardstown with a friend to meet two girls.
It was the prosecution's case that, later in the night, Dekker and Noone lured him to Tolka Valley Park where they stabbed him to death with a single blade from garden shears.
The only reason given for the attack was that earlier in the night Daniel was boasting about people he had fought. When he mentioned having fought one of Noone's relatives, Noone punched him in the face. Daniel's friends said the pair made up. But later in the night, according to interviews given by Dekker to gardaí, Noone decided he was going to give Daniel "a hiding".
Dekker said Daniel begged for his life but Noone insisted "he has to go" and stabbed him again and again.
He also told gardaí he had no idea how the body got to Rathfeigh but he suggested that Noone could have moved it.
The murder verdict means that the jury believed the prosecution proved beyond reasonable doubt that he aided and abetted Noone with the intention of killing or causing serious injury to the boy.