The National Transport Authority has said passenger numbers on public transport have increased as fares are being reduced as part of Government plans to help ease the cost of living and to encourage more sustainable travel.
It is the first time public transport fares have been cut nationally since 1947.
Fares on Dublin Bus, Luas, Go-Ahead Ireland and Iarnród Éireann's DART and Commuter services will drop by 20% for the remainder of the year.
Iarnród Éireann's InterCity and Commuter fares across the country are also being reduced.
In addition, young adults aged 19-23 will also now receive a 50% reduction in fares on all subsidised public transport by using an existing Student Leap card or applying for a new Young Adult Leap card.
Fares on Bus Éireann and Local Link services were reduced last month.
The NTA said that since then, passenger journeys have climbed by almost 10%.
NTA Chief Executive Anne Graham said she hopes that this pattern will continue across the country and that people will consider leaving the car at home at least once a week and use public transport instead.
However, the Coach Tourism and Transport Council of Ireland (CTTC) said the decision to exclude private bus companies from the 20% fare reduction risks putting hundreds of operators out of business while adversely impacting the future of Ireland's transport network.
It said the preferential treatment of State services raises questions around fairness as it will allow State-funded transport services to offer a significantly reduced rate to passengers.
The organisation said its members fear the loss of patronage could push hundreds of family-owned, private transport companies to financial ruin.
It has estimated that the total cost to the Government of including private operators in the scheme would be around €23.7m.
"The majority of our members operate regional and commuter services - and as a result of this exclusionary policy, many of our passengers are now being actively disadvantaged at a time when there is a cost of living crisis," said Chairperson of the CTTC, John Halpenny.
"The solution is straightforward - the Department of Transport must seek to include commercial bus operators in the 20% fare reduction scheme as a matter of grave urgency."