Strike action has begun at Bus Éireann over the implementation of a €5m cost cutting plan.
The action by members of the National Bus and Rail Union began at midnight.
Bus Éireann says the impact on services across the country is not yet clear.
It says intending passengers should check the company's website for details of the routes affected.
Management at the company is due to implement a €5m package of cost-cutting measures in line with Labour Court recommendations.
NBRU General Secretary Michael Faherty had said the union and Bus Éireann had publicly stated that they were willing to talk, but no such dialogue took place.
He said Bus Éireann workers are being made an example of in front of other trade unions as the Government tries to renegotiate the Croke Park II agreement.
Responding to the claim, Minister for Transport Leo Varadkar said the dispute was about saving services and jobs at Bus Éireann.
In a statement, Mr Varadkar said Bus Éireann was not under Croke Park II and its staff have not been subject to pay cuts or the pension levy imposed on public servants.
Andrew McLindon, a spokesman for Bus Éireann, said that it was proving difficult to plan ahead as the company did not know if members of SIPTU would pass the picket today.
He said the strike will cost Bus Éireann €200,000 a day.
Separately, private bus operator JJ Kavanagh said there is spare capacity on his buses and nobody will be left behind at a bus stop.
He said the company always has additional vehicles if required.
James McGinley of McGinley coaches in Donegal said not knowing if the strike is to go ahead is inconvenient and making it difficult to plan.
He said the company will have extra coaches on standby.