Barbara Walsh (33) was last seen more than 30 years ago at her home in Carna, Galway.
A team of technical experts, senior detectives and the Garda Technical Bureau have started a dig on land in Connemara near where the mother of seven went missing.
Sniffer dogs and excavators have also been brought into the search which is expected to last several days.
Ms Walsh was last seen after a house party in Rusheenamanagh area of Galway at around 4am on 22 June, 1985.
However, Gardaí were only notified of her disappearance a week later.
Gardaí decided to revisit the case and launched an investigation in January on this year.
They are currently following 112 lines of enquiry and 60 people who have been interviewed.
Ms Walsh was described as a dedicated mother, person full of “joy” and had a passion for ponies.
Gardai are appealing for anyone who may have any information about the disappearance of Barbara, to contact them at Clifden Garda station on 095 22500, the Garda Confidential Line on 1800 666 111 or any Garda station.