The family of 17-year-old Raonaid Murray are appealing for information on the 21st anniversary of her murder.
The Gardaí and Raonaid's family say they are continuing to work together in an effort to solve the case.
She was last seen alive at approximately 11.20pm on Friday September 3rd 1999 when she made her way home from Dun Laoghaire town centre.
Her body was found at Silchester Crescent in Glenageary at 12.33am on Saturday September 4th, less than 500 yards from her home.
In a renewed appeal, Gardaí say: "Many who were then Raonaid's age are now parents themselves with children, some of whom would now be close to Raonaid's age, and we would ask them to reflect now with the benefit of maturity and hindsight on any information which may be of assistance to the investigation.
"Despite a large number of suspects having been identified during the course of this investigation, there is in reality no prime suspect.
"The lack of a prime suspect largely arises from the fact that there has been no motive identified in this case.
"In over 4,500 witness statements there is not one word of enmity towards Raonaid".
They add: "If any person has any information which could assist in identifying a motive for the murder of Raonaid and/or if any person has any doubts about the truth of an alibi already provided, we would appeal for your immediate assistance.
"You may unknowingly be shielding a killer."
In a statement last year, her father Jim said: "Raonaid’s killer is free. That freedom mocks what should have been Raonaid’s life and mocks the horror of her death.
"To her killer we say come out from the shadows and own up to what you have done. Do the right thing and confess your crime."
They are appealing to anyone with information to contact the incident room at Dun Laoghaire Garda station on 01-666-5000 or 01-666-5012, the Garda Confidential Line at 1800-666-111 or any Garda station.
A reward for critical information received is available from Crimestoppers on freephone 1800-25-00-25.