Irish Rail, Dublin Bus and Luas have all sacked or sanctioned drivers over the past three years for failing alcohol and drugs tests.
Irish Rail said a minimum of 5 per cent (190) of all its 3,800 employees – across all roles – undergo random drug and alcohol testing every year.
There were also “for cause” tests carried out after any incidents, spokesman Barry Kenny said.
One train driver failed a test over the past three years, he added.
The employee was “subject to our disciplinary process” and “is no longer employed in a safety-critical role”.
“The test was positive for alcohol in a post-incident situation, arising from shunting out-of-service vehicles,” Mr Kenny said. The incident happened in 2018.
There were 220 tests carried out across all staff last year, 226 the previous year and 193 in 2017. There was no breakdown of how many drivers were among them.
Dublin Bus said all its internal testing for drugs and alcohol was carried out by the CIÉ medical department.
"Under the Road Traffic Act 2016 our bus drivers can be randomly tested for drugs and alcohol at mandatory checkpoints or if involved in a road traffic accident," said Dublin Bus spokeswoman Carol Donohue.
“We have had one failed test over the past three years as a result of an alcohol test post-incident.
“The relevant employee’s contract of employment was subsequently terminated.”
Ms Donohoe declined to detail any of the circumstances surrounding the incident or the number of random checks carried out by Dublin Bus on its drivers every year.
Separately, a Luas driver was also sacked over the past three years for failing a drugs and alcohol test.
Transdev, which operates the capital's light rail system, refused to answer any questions about the incident; however, Transport Infrastructure Ireland (TII) – the State agency in charge of roads and railways – confirmed the dismissal.
In response to a parliamentary question by Social Democrats TD Catherine Murphy, the TII's head of regulatory and administration, Gary Lynch, said: "One instance occurred in the past three years in which a Luas driver failed a drugs and alcohol test.
"The sanction imposed by the Luas operator, Transdev Dublin Light Rail, in such cases is dismissal from employment," he added.
Dervla Brophy, spokeswoman for Transdev, declined to answer a number of questions about the incident, saying: "We've no comment."
Bus Éireann said none of its staff had failed any such tests over the past decade.
It said all alcohol and drug testing on Bus Éireann staff was carried out by the CIÉ medical department.
There is a very significant obligation when you are carrying passengers on public transport
"Over the past 10 years, no Bus Éireann employee has tested positive under such tests," said spokesman John Sheridan.
“All staff have an obligation to comply with the company’s drug and alcohol policy.”
Mr Sheridan declined to give numbers for testing on staff.
In January Bus Éireann apologised after one of its drivers failed a Garda breathalyser test while in charge of a bus with passengers on board on New Year’s Day.
Ms Murphy said she would like to see more transparency in relation to how alcohol and drugs testing is handled by public transport operators.
“There is a very significant obligation when you are carrying passengers on public transport that you carry them safely,” she said.
“It would be useful and important to see where alcohol and drugs testing fits into each of the operators’ risk assessments: whether it’s random or routine; if it’s internal, what kind of a code does it come under.
“There is a deficiency of information. But I don’t think there is very significant evidence that there is a wholesale problem and that has to be acknowledged at the same time.”
Ms Murphy added that private operators of public transport needed to be equally opened to the same scrutiny.