The body of Marilyn Rynn was found near her home in Blanchardstown in Dublin in early January 1996, 16 days after she went missing.
The 41-year-old, who worked at the Department of the Environment, was reported missing days after attending her work Christmas party in Raheny on December 21 1995.
That was the last time she was seen alive.
Her family reported her missing on St Stephen's Day after she failed to show up on Christmas Day.
It was 7 January 1996 before her body was discovered just a five-minute walk from her home.
A murder investigation began and as part of that, DNA samples were taken from hundreds of men.
David Lawler was one of the men who volunteered a blood sample.
Despite being exposed to the elements, the cold weather at the time ensured the DNA on the body survived and gardaí were able to get a match.
David Lawler was arrested and admitted to the killing. He was jailed for life in early 1998.
He was released from prison earlier this week.
Author and crime journalist Barry Cummins wrote about the murder of Marilyn Rynn and conviction of Lawler in his 2004 book 'Lifers’.
He told Behind the Story that Lawlor had followed the 41-year-old on her walk home.
"By his own admission, he followed Marilyn from Blanchardstown village," he said.
"Their paths crossed by chance; Marilyn didn’t know she was being followed, she hadn’t seen this man at all but he fixated on Marilyn.
"In an area, a shortcut to her home known as The Tunnels, that’s where he attacked her."
Mr Cummins said the investigation was initially told of sightings of Marilyn, which later turned out to be incorrect.
"These wrong steers did delay things in a particular way and put the focus actually in the city centre for a while," he said.
"This was a time when there was very little CCTV footage, certainly not like it is today in Dublin city.
"There was footage to show Marilyn going for the Nitelink, but they had to establish had she got on the bus?"
Mr Cummins said once gardaí established Ms Rynn had got close to her home, they focused on the men in the area.
"354 men were asked would they volunteer a blood sample," he explained.
"David Lawler was one of those who volunteered to give his DNA."
Mr Cummins said Lawler gave a mix of "fact and fiction" to the gardaí, after having done "his own amateur detective work" online.
"He was trying to find out was his DNA still to be found, or would it be diminished by the time Marilyn’s body was found.
"But what he wasn’t factoring in was the freezing temperatures… [which] maintained the DNA.
"This was the first mass DNA trawl by gardaí in any murder investigation."
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