Thousands of bus passengers in Dublin were affected by an unofficial strike by Dublin Bus drivers in the Ringsend depot this morning.
Routes affected included all 15s, 49, 65, 77 and 150 serving Rathmines, Templeogue and Tallaght.
A spokeswoman for Dublin Bus said the drivers returned to work at 10.30 this morning "without any commitment from the company". She said an investigation is under way into why the unofficial action took place and what led to it.
"Correct procedures weren't followed. A full agreement exists between the company and unions on procedures and policy and these were ignored", she said.
Officials from the National Bus and Railworkers Union (NBRU) are meeting Dublin Bus management later today to discuss issues behind the unofficial action.
The union said a driver was urinated on by a passenger in the west Tallaght area last week and had been told to go on sick leave. NBRU executive member John McGrane said the driver had wanted to go back to work instead of receiving sick pay.
"He is married man with a mortgage and he wanted to work his weekend," he said.
Mr McGrane said his attempts to broker a deal with Dublin Bus last night had failed and that the unofficial picket had been mounted as a result. "A lot of drivers have been affected by anti-social behaviour and they can't be losing a liveihood because management want them to go on sick leave", he added.
Speaking in Tallaght this morning, prior to the launch of the second Luas light rail line, the Tánaiste, Ms Harney, said drivers should follow "proper procedures" to resolve any dispute.
"Wildcat action is unacceptable. The people losing out most are the travelling public," she added.