There is "no doubt" commuters will face further chaos in the Bus Eireann dispute, a union has warned.
Unannounced secondary pickets in support of the ongoing Bus Eireann walkout crippled Dublin Bus, Irish Rail and Dart services yesterday, catching tens of thousands of people by surprise.
Many were left out of pocket after seeking alternative transport, while retailers said they lost tens of thousands of euro from the "illegal" action, which even drew disapproval from union leaders.
Speaking on RTE's Morning Ireland, Willie Noone, transport sector organiser with Siptu, said he had no doubt that another flash strike would happen again as "feelings of frustration among workers are high".
"It's clear from people on the picket lines that this action is more than likely going to happen again," he said.
Schools could be next to be hit by the strikes, with school bus drivers announcing they are to hold a ballot for industrial action.
They feel a threat to their jobs resulting from the wider crisis in the company.
The Siptu National School Bus Drivers Committee also decided to begin the process of balloting its members for industrial and strike action in sympathy and support of their colleagues in Bus Eireann.
"Although they are employees of Bus Eireann, these workers have to date not been party to the ongoing dispute at the company," Willie Noone said in a statement.
"However, it is now known that their livelihoods will also be adversely affected in the circumstances that their employer is not able to continue trading.
"This leaves our members with no other option but to consider industrial action to protect their jobs and the services they provide."
Bus Eireann said escalation of the action in an illegal manner would not resolve the matter, adding it "can only engage on the basis of good faith".
Another union, the NBRU, said it sent a strong message to members about the wildcat action. It has reiterated the need for the current dispute to be resolved.
Siptu transport division organiser Greg Ennis said the union "did not organise and does not condone" the unofficial action, which disrupted rail and bus services.
Bus Eireann workers placed pickets on Irish Rail and Dublin Bus depots in the early hours in an unexpected move.
The strike, which has been running for a week, is running indefinitely and centres on cost-cutting plans set to be introduced by the company.
Transport Minister Shane Ross said he was encouraging Bus Eireann staff to return to discussions in a bid to end the strike. However, he reiterated that he would not be intervening in the dispute.
He described the unofficial action as "appalling".
"It's just not on and the law will have to take its course in cases like this because this is not something which I'm sure ordinary bus workers wanted to happen," he said.
"The travelling public did not want this to happen."
He said he was confident that the parties would return to talks but said he would not be intervening if they chose not to.
"I'm not going to intervene myself in an industrial relations dispute. This is something which the parties have got to sort out themselves," he said.