Sandra Collins was last seen at a takeaway in Killala, Co Mayo, at 11.15pm on Dec 4, 2000.
Her chip bag, along with her fleece jacket, were found on a nearby pier four days later.
Gardaí suspected that the scene had been staged to look like a suicide. Ms Collins’s body has never been recovered.
Last night, detectives arrested a man in his 40s in the Ballina area. He was held overnight at Ballina Garda Station and is due to appear before a special sitting of Ballina District Court at 11am, charged in connection her killing.
There have been a number of arrests in the case, including a 21-year-old woman who would only have been 11 years old at the time of the killing.
She and a 46 year-old man and a 58-year-old man were all detained and questioned in Mar 2011.
It was thought the woman, who was released without charge, had been arrested because she had information about what happened to Ms Collins. The 58-year-old was also released without charge.
When the 46 year-old was released, a file was sent to the DPP. He was arrested again in Dec 2011.
The arrests followed a fresh appeal for information in Dec 2010. That led to a number of new witnesses coming forward, who spoke of a white van at the time of the disappearance.
Gardaí, along with Crimestoppers, then made a further appeal in March, asking for the public’s help in identifying the van.
The carer, just a few days short of her 30th birthday, had walked into Killala for groceries for her disabled aunt, before she was spotted at the Country Kitchen takeaway.