Speed cameras around the country's roads are so effective that they cannot pay for themselves, the Garda chief has claimed.
The privately-operated GoSafe mobile camera vans will cost the taxpayer 12 million euro more than expected this year, a parliamentary watchdog has been told.
Quizzed about how Garda bosses got their figures so wrong, acting force chief Noirin O'Sullivan said the road safety cameras were so good at deterring motorists from speeding that the number of fines are a lot lower than forecast.
It was planned that the money raised from fixed penalties resulting from detections would pay for the system, introduced in 2009.
"In terms of value for money, certainly in terms of saving lives, it is very difficult to speak about any loss of life or serious injury in monetary terms," said Ms O'Sullivan.
"Nevertheless one of the reasons the receipts are down is that the GoSafe cameras and the safety cameras are actually achieving their objective of increasing compliant behaviour."
She added: "The objective was always to reduce speed related collisions and as a consequence to ensure less lives were lost."
More than 500 locations targeted by the mobile cameras are published on the Garda website.
GoSafe won the 80 million euro five-year contact to operate them, which is coming to an end towards the end of this year.
Most recently reported accounts show the firm making almost 50,000 euro a week profits on the speed camera operation.
When Garda chiefs negotiated the deal, they were working on the assumption of only half of all motorists complying with speed limits at the designated spots on the nation's road network.
This was based on international comparisons, according to the force.
However, between 86% and 99% of drivers are complying with the law in Ireland at the speed camera locations, identified because of high numbers of serious accidents.
Superintendent Con O'Donohue, of the Garda National Traffic Bureau, said the controversial mobile speed camera operation was the first of its type in any jurisdiction.
The force was "moving into the unknown" in estimating costs, he told the parliamentary committee.
"It was difficult to bring in an estimate because of it was unique," he said.
Supt O'Donohue added that when the contract is up for renewal in a few months time everything will be "back on the table" in terms of its value for money.
Earlier this year, a judge described the failure of GoSafe to successfully prosecute alleged speeding motorists as "a complete waste of public money".
Judge Patrick Durcan was referring to several failed prosecutions involving the private company.
However, Ms O'Sullivan said the safety cameras had prevented 23 deaths of the roads.