Hundreds of thousands of people face travel disruption this morning as drivers at Bus Eireann and Dublin Bus go on strike.
The row centres on plans to sell off one in 10 bus routes to private operators.
Today's action is going ahead after 11th hour talks at the LRC broke down yesterday.
Both companies say they will take legal action against the unions and will go to the High Court today to recoup the revenue lost as a result of the strike. The companies say they believe this is not a legal strike.
Officials attached to SIPTU and the NBRU say they heard nothing new from Bus Eireann or Dublin Bus during yesterday afternoon's talks.
As a result, they are going ahead with a 48-hour industrial action that could affect up to one million passengers.
The action is expected to cost Bus Éireann and Dublin Bus a combined €20m.
Further strike days are planned by both unions on May 15 and 16, while the NBRU is proposing another three days of action on May 29, 30 and 31.
Striking workers will this morning gather at 11am at Parnell Square in Dublin and march to the Dáil.
Gardaí say bus lanes will NOT be open to motorists, as they will be reserved for taxis, cyclists, private bus companies and the emergency services.