Trevor Deely, 22, was last seen walking home from a Christmas party in December 2000.
A search of a site in Chapelizod in Dublin started early on Saturday morning, Irish national broadcaster, RTÉ reports.
The area is three acres of Woodlands beside the R112 which runs down to the River Liffey.
Gardaí said the search will involve excavations and is expected to take a number of weeks.
Earlier this year a team of six detectives began conducting a complete review of Mr Deely's disappearance.
CCTV footage from the night he went missing was enhanced using specialist technology.
It shows Mr Deely talking to a man dressed in black outside the Bank of Ireland on Baggot Street, where he worked.
It is understood he was returning to get an umbrella after the party.
Detectives released the footage in April and appealed for the man to come forward.
They believe he was the same man captured on CCTV following Mr Deely, who was from County Kildare in the Republic of Ireland, a short time later on Haddington Road.
On Saturday gardaí said the man seen in the CCTV has not yet been identified and they renewed their appeal for him to come forward.

The 26-year-old England tight-head prop, who was sent off, pleaded guilty at a disciplinary hearing on Tuesday.
Brookes will miss Saints' Premiership game at Leicester on Saturday and their Champions Cup game with Leinster.
Newcastle lock Calum Green, 26, has also been banned for a week after admitting striking with his arm.
Green, who will miss Falcons' home game with Harlequins on Sunday, struck Brookes with his forearm during the same game and was cited by the citing commissioner after the incident was missed by referee Andrew Jackson.
Brookes was dismissed in the 38th minute of Saints' 22-16 defeat when he charged into the ruck and struck the head of Newcastle hooker Scott Lawson with his shoulder.

Avon and Somerset Police closed the Lower Bristol Road in Bath at Rackfield Place outside the First depot after the crash at 03:00 GMT.
The A36 road closure, which was lifted at 14:00 GMT, had caused disruption and delays to commuters and bus services during rush hour.
Investigations are under way into how the collision happened.
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First said that while parts of its services had resumed a normal schedule, it would take a few hours for all services to return to normal.
Managing director James Freeman said: "We are hoping we will have a full service running in time for the evening commute.
"We would like to thank the public for their support and understanding and staff for going beyond their call of duty to help manage the situation and recovery."

Alexander Reid repeatedly told Department of Work and Pensions staff in application forms and at interviews that he was single.
But in reality he was living with his wife Kathleen Reid, despite having claimed to be separated.
Reid was found guilty following a trial at Dundee Sheriff Court.
The 59-year-old, from Dundee, had denied a charge under the Social Security Administration Act that he fraudulently claimed employment support allowance and income support totalling £39,808.
Defence solicitor John Boyle asked that Reid be spared jail and given a community payback order as an alternative to a prison sentence.
Sheriff Tom Hughes told Reid: "Because of the sum of money involved a custodial sentence is the only option."
Gardaí (Irish police) investigating the disappearance of a man in Dublin almost 17 years ago have begun fresh searches.
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Northampton's Kieran Brookes has been banned for two weeks after admitting charging into a ruck without using his arms in Friday's loss to Newcastle.
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A woman has died after her car crashed outside a bus depot trapping 150 buses inside.
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A man who fraudulently claimed nearly £40,000 in benefits over the course of almost 11 years has been jailed for 18 months.