Families of missing people are being urged to give DNA samples to try to solve hundreds of cases, some of which date back nearly 70 years to 1951.
Gardai said 813 cases of people who disappeared in Ireland have not been closed.
A national DNA database set up five years ago has been credited with bringing a number of long-term cases to a close.
It relies on a DNA sample from a missing person's family.
"The majority of persons reported missing return soon after their disappearance without suffering any harm," a spokeswoman for the garda Missing Persons Bureau said.
"A small percentage may come to harm or be the victim of crime.
"An investigation will remain open until the missing person has been located, irrespective of the time passed."
The unit has called on families to provide DNA.
"Parents, siblings and children of missing persons can provide a DNA sample, which will be uploaded to the Missing Persons Database," the spokeswoman said.
The database, managed by Forensic Science Ireland (FSI), has been hailed as "one of the most important crime-fighting tools introduced within the state in recent times".
Operation Runabay was launched in 2017 with the aim of collecting DNA to identify remains found along the west coast of Britain that may belong to people who went missing in Ireland.
Last year, the operation helped solve the case of Conor Whooley (24), who disappeared from Dublin in August 1983. His remains were identified in a graveyard in Wales.
In 2018, the database helped to identify five missing people, bringing "much-needed closure to their families and friends".
The FSI developed new technology that helped it identify DNA from bones, including those recovered from water.
This meant that bodies found years earlier, but which could not have DNA taken from them, could finally be identified.
The body of Aengus 'Gussie' Shannon (20), who went missing from Limerick in 2000, was identified 18 years later.
Bones were found in the Shannon in 2001, but it had been impossible to take a DNA sample from them until 2018.
In November that year, the technology successfully identified the remains of two more long-term missing persons.
Margaret Glennon, a 49-year-old mother-of-four, went missing from her home in Baldoyle in 1995.
Remains discovered by a JCB operator in Malahide in 2014 were confirmed as hers in 2018, using new DNA technology.
The disappearance of James Gallagher (18), from Cabra, who went missing in 1999, was also solved. It was confirmed that remains discovered in Dublin in 2002 were his.
The FSI also worked with two police forces in 2018 to identify the remains of two other missing people.
A body found on a North Wales beach 33 years before was finally identified as that of Joseph Brendan Dowley (63), from Kilkenny, who was last seen boarding a ferry from Ireland to Britain in October 1985.
Paul Shine-Dixon, a 28-year-old father-of-three from Finglas, went missing while on his way to meet his partner in Barcelona in 2009.
He was travelling through Europe by train when he got off in the French city of Perpignan on May 2.
His remains were discovered buried near a canal in Perpignan in 2018.
Garda used the national DNA database to work with French authorities to identify his remains.
In December 2018, the nat- ional forensic science lab used a National Missing Persons Day event at King's Inn in Dublin to collect DNA from families of missing people.