Prison officers who catch visitors bringing drugs into prisons are powerless to detain them, and have no alternative but to allow them leave the prison.
People caught by prison wardens carrying drugs often flee the prison before gardaí arrive.
In some cases people identified as carrying drugs by sniffer dogs, are allowed proceed with the visit behind a screen without being searched.
A spokesperson for the Irish Prison Service said: "We can’t grab them and search them… if someone gets past the dog and there’s an attempt to pass something, the gardaí are called.
"And everyone who visits, we have their contact details so the whole thing is passed on to the Gardaí. Our priority is to make sure nothing comes into the prison. It’s not our authority to catch or prosecute people for possession of drugs because we’re not entitled to."
Irish Prison Service director Brian Purcell said there would be a "significant impact" on drugs coming into jails over the next six months with the full roll out of a 30-dog, drug-detection unit across jails.
The deputy general of the Prison Officers Association, Eugene Dennehy, said: "They’re walking out of prison because we don’t have the powers to deal with them and that’s something that needs to be looked at."