Gardaí are searching any premises linked to the 39-year-old officer. They are trying to establish where he obtained the drugs and who he had been supplying.
Drugs Minister Pat Carey said he was “disturbed” by the incident.
“It is unpardonable. I condemn it without reservation and I certainly complement the gardaí and the prison authorities for taking a firm line,” said Mr Carey.
Jim Mitchell of the Prison Officers Association (POA) said: “Such persons will have no support whatever from this organisation — the actions of the few can bring disgrace and embarrassment to all of the hardworking prison officers around the country, who would have no tolerance of or take part in any form of illegal activity.”
It is understood Prison Service intelligence officers, attached to an Operational Support Group, planned to conduct a body search on the prison officer after carrying out inquiries.
The prison officer, from the northside of Dublin, was patted down after he walked through the security scanner at Mountjoy Prison.
During the search, they discovered something strapped on his inner thigh. Garda sources said it contained a “selection box” of drugs, including 450 grams of cannabis, 310 D10 tablets (a minor tranquilliser) as well as smaller quantities of herbal cannabis, cocaine and heroin.
Gardaí from Mountjoy Garda station arrived within minutes and at 8.20am arrested the officer under section 4 of the Misuse of Drugs Act. The drugs were seized and samples were sent for forensic examination.
Following inquiries, detectives searched the prison officer’s car, located in the prison grounds. There they uncovered a second quantity of drugs, with an estimated street value of €5,000.
Sources said they will investigate whether the prison officer was acting under duress or for profit.
A spokesman for the Prison Service declined to comment on the specifics of the case, saying it was now under garda investigation.
“The Prison Service is determined to clamp down on all avenues of drug trafficking in prisons. This would be a rare occurrence.”