One of the two teenagers who murdered Ana Kriegel was assaulted in a planned attack by fellow detainees in a centre for young offenders.
The convicted murderer, known as Boy A during the horrific murder trial, was assaulted by two fellow inmates in the Oberstown Detention Centre on Sunday night.
The 15-year-old was set upon while he watched television and received several kicks and punches to the head before staff intervened to break up the assault.
A source said Boy A, who was sentenced to life in prison for the murder of 14-year-old Ana, received minor injuries from the assault due to the early intervention by staff.
The source said the attack on the teenager, who was convicted of the murder and sexual assault last year, "appeared to be planned".
Local gardai from Ballbriggan in North Dublin are examining CCTV footage related to the incident which was recorded in the detention centre.
In a statement, a spokesperson said: "An Garda Siochana are investigating an alleged assault that occurred at Oberstown Detention Campus on Sunday 24th May 2020."
Ana Kriegel was found murdered in an abandoned building near her home in May 2018.
She had been sexually assaulted before she was beaten to death.
She had been reported missing by her parents, Patric and Geraldine Kriegel, three days earlier.
Two 13-year-old boys were arrested and charged with her murdered.
Both denied responsibility for the murder which shocked the country.
After a lengthy trial, the two boys were found guilty of Ana's murder by a jury in June last year.
They were the youngest people to be tried and convicted of murder in the history of the State.
Boy A was also convicted of aggravated sexual assault involving serious violence and was sentenced to life imprisonment for the murder, with a review after 12 years.
He was sentenced to an additional eight years for aggravated sexual assault.
After the trial, it emerged there were 12,000 mostly pornographic images found on two mobile phones belonging to Boy A after detectives searched his home. Among the online searches on the convicted killer's phones were "child porn", "animal porn", "horse porn" and "dark web".
The jury was never told about the pornographic images as Mr Justice Paul McDermott ruled they could not be produced in evidence, saying that "relevance" was lacking.
Judge McDermott said the jury had ample evidence of the acts allegedly being perpetuated to give them a complete understanding of the case.
Until last year, Boy A had denied murdering Ana despite damning evidence produced during the trial.
In the months since his sentencing, Boy A has been detained at Oberstown and has been interviewed by medical professionals.
It was during these interviews that Boy A finally admitted his responsibility for Ana's death in May 2018 but continued to deny the sexual assault.
The other teenager, Boy B, who was also unanimously found guilty of Ana's murder and detained in Oberstown, is appealing his conviction and recently lodged appeal documents to the Court of Appeal.
He has denied his involvement in the murder in accounts given to professionals while in detention.