A €100,000 reward is being offered for information on Trevor Deely's disappearance as part of a renewed appeal.
An anonymous donor has offered the reward through Crimestoppers.
It comes as new CCTV footage has been released showing a man dressed in black acting suspiciously on the night Mr Deely disappeared outside the rear entrance of Bank of Ireland Asset Management offices on Wilton Terrace in Dublin.
Mr Deely, from Naas in Co Kildare, was last seen walking home from a Christmas party in Dublin city centre in the early hours of 8 December 2000.
Gardaí have discovered that the man they are now trying to trace waited outside Mr Deely's workplace in the early hours of that morning.
The new development, which gardaí believe is significant, came about as a result of technological advances in the examination of CCTV footage.
Detectives at Pearse Street Garda Station never closed the file on Mr Deely's disappearance and hope this new information will lead to a breakthrough in the case.
Numerous appeals for information over the years failed to establish what happened to him.