Three people have been arrested in connection with the murder of Marie Gilmartin (35) in 1993.
The arrests were all made in the Portlaoise area this morning.
A man and a woman, both in their 60s, are being held at Tullamore Garda station under the Offences Against the State Act.
A second man (42) is being held at Portlaoise Garda Station.
Marie Kilmartin, who lived in Portlaoise, was last seen alive on the morning of Friday, December 17th, 1993, in Laois Shopping Centre in the town.
Her body was found in a bog at Pims Road, about half a mile off the Mountmellick-Portarlington road, six months later. She had been strangled and a concrete slab had been placed on her chest. Detectives believed that one or more individuals with local knowledge of the midlands may be responsible for her murder.
But despite an ongoing investigation, a media campaign and a reward offered for information leading to the discovery of her killer, gardaí have been unable to move the case forward.