The 15-year-old had been charged with having sex with a 14-year-old girl in 2006.
The case attracted much notoriety in the media and became known as the 'Romeo and Juliet' case because the law allows the prosecution of teenage boys for having underage sex with teenage girls but prevents the prosecution of the girls.
The man, who is now 21, is from Co Donegal but cannot be named. He pleaded guilty to the defilement of a child when he appeared at Letterkenny Circuit Court.
The man admitted the charge, which took place at a vacant house in 2006.
The court heard how the accused and another two boys and two girls entered the house after a ceili on August 5, 2006.
The girl admitted she took part in various acts of intimacy, including performing oral sex on the boy. However, she denies she ever consented to full-sex with the boy.
The gardai were informedwhen another girl told her father and he, in turn, informed the local school principal.
A full investigation was launched and the girl was taken to the Rotunda Hospital in Dublin for treatment.
The court heard the young man had admitted to defiling the girl but claimed the sex was consensual.
The court heard that the girl had suffered from depression because of the incident and was on medication.
Barristers for the young man said the case had been hanging over him for six years as appeals were made to both the High Curt and the Supreme Court.
Judge John O'Hagan said the case involved a couple of children who got together and their hormones ran away with them.
He said the Oireachtas had clearly recognised that this was not a "sexual predator" type crime. The judge said that perhaps he was too old to answer some of the questions thrown up by the case.
"The circumstances surrounding how it happened are quite different than perhaps what some of we older generation might have experienced.
"Young people have different views about experimenting with their bodies," he said. He added that he was not in any way blaming the young girl or suggesting she did anything wrong.
"On the other hand I am not suggesting (the man) did anything wrong in the heat of the circumstances of the moment."
Judge O'Hagan said that he had no plans to impose a custodial sentence on the young man.
Instead he sentenced him to six months in prison, suspended for 12 months.