The money in the bag was the suspected proceeds of an armed robbery at a post office in Lusk, north Co Dublin, last January.
Armed raiders made off with 26,000 after holding up staff at the premises, but the haul was recovered soon afterwards.
Detectives on an anti-crime patrol responded to an emergency call about the raid and intercepted the gang as they were making their escape.
After the cash was taken by gardai to Balbriggan station in Dublin, it was bagged and tagged after being photographed for use as evidence in a subsequent criminal trial.
The money was then placed in a safe at the station.
However, when the owner of the premises called to seek the return of the money, gardai discovered a small tear in the side of the bag.
The contents of the bag were checked and it was discovered that 9,000 had disappeared.