The head of the National Bus and Rail Union (NBRU) has said some of its members are 'putting themselves in the middle' of situations where people do not wear masks.
The wearing of face masks or face coverings will be compulsory on public transport from Monday.
People who do not comply could be fined €2,500 or face six months in prison.
But general-secretary of the NBRU, Dermot O'Leary, told Newstalk Breakfast compliance is varied.
"It's mixed out there.
"Newstalk itself and many of the shows on Newstalk's platforms have been reporting that there's been initially slow uptake from people.
"Now it has improved over the last number of weeks, but we're still not at a stage where you have anything near 100%.
"In actual fact, in Dublin, it's been as low as 30% or 40% in some cases.
"It'd be higher in the country and in rural areas - but it's still not at the levels you expect it to be from a compulsory point of view."
"And despite the fact that I'm saying on behalf of my members that we won't be policing it, there are some of our members putting themselves in the middle - as it were - and trying to get people to wear a mask.
"There is that bit of confrontation out there that we're trying to avoid, and that's the reason why we're saying we're not going to police it."
He said plans to call the Guards for such situations "doesn't work in practice".
"You have a situation currently where if a bus driver drives his bus and there is something going on behind him, that he's aware of - an altercation - that he does call in control and the Gardaí can be called.
"That's a rare occasion that will happen - for that to happen on a regular basis, you can imagine how buses will be held up - or trains in fact could be held up leaving main stations".
He also said that Gardaí "don't have the resources that they should have to police this".
"I feel certainly that from Monday there should be at least in the initial stages types of spot-checks by Gardaí to demonstrate to people that you can and you will be fined".
"It's a good move for the public transport, and the reason we've been calling for it for quite a while now is to restore confidence for people to travel on public transport, in fact.
"We want public transport to be central to the recovery of the economy - and we don't want to see a situation where the motorcar will be coming back into our cities in large numbers."
"There are issues around who's going to be policing this, and that's where we're going to have a problem from Monday.
"Whilst the legislation is welcome and the concentration by Government at long-last - we've called for this since the 1st of May, in fact."
Mr O'Leary said they have been corresponding with both the National Transport Authority and the operating companies since then asking "the pertinent question 'who's going to be policing this - and we still have no answer to that".