More than 800 missing persons cases remain open investigations on garda files.
The figure was announced as the country today marks Missing Persons Day, an annual event of commemoration that also recognises the trauma of families and friends left behind.
One woman who takes much comfort from Missing Persons Day is Ann Boyle, whose daughter Mary was just six when she vanished on the Co Donegal and Co Fermanagh border on March 18, 1977.
"Mary would have been 50 this summer, and her birthday is always a hard day," said Ms Boyle.
"I get great comfort from meeting other families on Missing Persons Day and our shared experience helps us deal with our loss.
"We're all in the same boat and we all know what each other are going through.
"We are an unusual club of people from all over the country all linked by an awful thing we share in common - someone belonging to us has disappeared.
"My granddaughter will be helping me access the event this year on a computer because the usual gathering was postponed because of Covid. It is a very important day."
Ms Boyle said she still holds out hope that her daughter Mary will be found.
"After 43 years I think Mary is the longest-missing child in the country," she said.
"I still think of her as the special little girl I remember but I do wonder too what she would be like now and where in her life she would be.
"My husband Charlie died 15 years ago not knowing where Mary is. Her disappearance took a toll on his health and on all of us, and I still hope she will be found before I die."
Mary was last seen at 3.30pm on March 18, 1977, near her grandparents' rural farm in Cashelard, near Ballyshannon, Co Donegal.
The family had gone to the Boyles' maternal grandparents' house on St Patrick's Day from their home in Kincasslagh in The Rosses.
Carmel Griffin, who works with the Missing Persons Bureau of An Garda Síochána, says Missing Persons Day is vital for families and recognises the lasting sense of loss of not knowing what has happened to a relative.
"That can be so evident and raw and it is important to recognise the lasting loss," she said. But Ms Griffin also stressed that even after years of waiting and wondering there can be breakthroughs.
"People have got in touch with us who have been missing for substantial periods of time with a wish to send a message back to their family that they are alive," she said.
"They may not want to establish direct contact, but even knowing the person you are so concerned about is alive can be of great comfort.
"So if you are a person who has walked away from your life and not looked back then you can get in touch with us.
"You will be listened to with sensitivity and without judgment, and your family can be told whatever message you want to send."
She also encouraged families of missing people to have their DNA samples logged with the DNA Database.
In future if any discoveries of remains are made - or if a person is found and their identity needs to be verified - their DNA can be cross-referenced with the database.
Under normal circumstances Missing Persons Day would be marked by a ceremony and a gathering of families to focus attention on unsolved missing persons cases and provide information on support services.
Covid restrictions mean today's event will be online.
The Department of Justice asked independent filmmakers to record interviews with families of missing people. It recorded a piece of music in the Garden of Remembrance.
These, along with recorded messages from Justice Minister Helen McEntee, Garda Commissioner Drew Harris and a representative from Forensic Science Ireland, will be played on the department's Twitter, Facebook, and YouTube channels.
The Garda Missing Persons Bureau can be contacted on 01 666 9476 or at firstname.lastname@example.org