A judge has directed that Michael Barry be acquitted on a charge of murdering Toyosi Shittabey in April 2010.
Toyosi, who was 15 and originally from Nigeria, died after being stabbed in Tyrrelstown, Dublin 15.
Michael Barry, 26, of Pigeon House Road, Ringsend, Dublin 4, had pleaded not guilty to the murder.
Toyosi and four of his friends were on their way back home from the National Aquatic Centre when the stabbing happened.
Judge Barry White directed there was no evidence to suggest Mr Barry knew his brother Paul was carrying a knife when he drove him to the scene where the teenager was stabbed.
The judge also cited the failure of a crucial witness to give evidence and the fact that Paul Barry was now deceased.
The trial had previously heard there was no suggestion Michael Barry had carried out the act of killing Toyosi, but that he had driven his brother to the scene where it was claimed that Paul Barry stabbed the teenager.
The judge directed a verdict of not guilty be issued against Michael Barry, saying: "the interest of justice may not be well served" if this were not the case.
At the start of the trial earlier this month, the prosecution said it was seeking a conviction of murder as Michael Barry had, they said, rendered it possible for his brother to carry out the act of stabbing the 15-year-old.
Counsel for the prosecution told the opening hearing that on 2 April 2010 the victim was with a "group of five black youths and four white females".
One of the girls asked for the light of a cigarette from Paul Barry. A row then ensued with "racist undertones" and name calling. The fight later broke up.
Believing a phone was taken by the group, Mr Barry and his brother Paul pursued them in a car.
They encountered them at a roundabout in Tyrrelstown.
The court was told Paul Barry, holding a knife, approached one of the youths.
Shittabey went to his assistance at which point Paul Barry turned around and, the prosecution said, the knife "ended up in the chest of the deceased".
Paul Barry went back to the car where, it was alleged, Michael Barry had remained and both men drove off.
Today, Judge White directed a not guilty verdict be given, saying during the course of the trial the jury had heard from eight of the young people present on the day.
However, one witness now living in England had "refused to co-operate" and while the judge did not know what his evidence may have been, the prosecution had indicated it was "important".
"In circumstances where the accused man's brother has died and with the refusal of a witness to co-operate, the interests of justice may not be well served in this case" were it to continue, Judge White told the jury.
Michael Barry was acquitted and later left the courtroom.