People are required by law to wear a face-covering on public transport from today.
Those who do not face a fine of up to €2,500 and the prospect of a six-month jail sentence.
Drivers can request people wear a face covering and can refuse people entry or order them to leave.
Gardaí could also be called to deal with any issues.
Dermot O'Leary, General Secretary of the National Bus and Rail Union, says members cannot be expected to enforce the rules alone.
"There is confusion now in relation to who's going to be policing this," Mr O'Leary said.
"The frontline transport staff that we represent will not be policing this. In other words, we're still confused in relation to who actually will be policing this new law. My understanding is the gardaí have no direct role in this.
"We're a bit confused, and again the reason for our confusion is because we haven't been at the table where the decisions were made."
