People are required by law to wear a face covering on public transport from today.
Those who don't face a fine of up to 2,500 euro and the prospect of six-months jail time.
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 can't be expected to enforce the rules alone: