Commuters risk a €2,500 fine or six months in jail for not wearing a face mask on public transport from today - but the new law is mired in confusion over how it will be enforced.
It is now an offence not to wear a face mask on a bus or train in a bid to curb the spread of Covid-19.
However, the National Transport Authority (NTA) could not say who would be designated to enforce the rule at busy railway stations.
Bus drivers have been told by their union they are not compelled to take on the role of demanding the public's compliance.
The lack of clarity over how it will be implemented is expected to lead to widespread flouting of the law.
The news comes amid concern as the number of new daily cases of the virus remains higher than in recent weeks, with another 17 people diagnosed with the infection yesterday.
It means that in the space of just four days, 90 new cases of the virus have been detected, as the impact of travel-related infection and virus spread among young people enjoying house parties and pre-Covid-style socialising takes hold.
The Department of Health, which brought in the new mandatory face mask regulations, signed off by Health Minister Stephen Donnelly, referred all queries on enforcement to the Department of Transport yesterday.
Earlier, Dermot O'Leary, of the National Bus and Rail workers' Union (NBRU), said: "Frontline workers should not be put in a position of potential confrontation with passengers.
"We support the wearing of masks, but it has been introduced without consulting with us.
"It is distasteful that pressure is now put on workers who were at the centre of supporting the public during lockdown.
"A bus driver was killed in France when he got into an altercation with a passenger over the wearing of a mask and we don't want that to happen here."
The NTA said the regulations allow for "any officer, employee or agent" to enforce the law, but could not say what would happen if someone bought a rail ticket at Heuston Station today and did not wear a mask.
The Department of Transport, in response to questions, said last night that the Department of Health was the lead agency for Covid-19-related regulations and public health guidelines.
"In circumstances where a non-compliant passenger, without reasonable excuse, fails to accept the refusal or comply with a relevant person's request, members of An Garda Siochana may be called to assist," it said.
"An Garda Siochana will continue its graduated policing response based on its tradition of policing by consent.
"This has seen gardai engage, educate, encourage and, as a last resort, enforce. That approach will continue in assisting the enforcement of these regulations.
"Where potential breaches of the public health regulations are identified, and where a person does not come into compliance with the regulations, a file will be submitted to the Director of Public Prosecutions."
Under the regulations, people are exempt from wearing a mask if they have a reasonable excuse, such as where they cannot put one on, wear or remove a face covering because of any physical or mental illness, impairment or disability, or without severe distress.
A transport employee can refuse to admit a person without a face covering on to a bus or train. They can also demand that a passenger remove themselves if they are found not to be complying.
Before this happens, they have to give the passengers an opportunity to "provide a reasonable excuse" for not wearing a mask.
Meanwhile, a new poll today shows that almost three in four people are likely to get a Covid-19 vaccine if one is found.
The Ipsos MRBI for the Irish Pharmaceutical Healthcare Association showed 52pc were very likely to get a Covid-19 vaccine, while 21pc were fairly likely. Almost one-fifth, or 17pc, were unlikely to get the vaccine, while 10pc were unsure.
Over four-fifths, or 81pc, of people believe a Covid-19 vaccine will be found. However, most of those - 62pc - believe it will be next year before one is available to the public. Just 11pc believe a Covid-19 vaccine will be available this year.