Iarnród Éireann staff and school-bus drivers have voted in favour of industrial action in support of Bus Éireann workers, if they resume their strike.
The move follows an earlier vote by Dublin Bus workers in favour of sympathy strike action.
Staff in Bus Éireann are currently considering Labour Court proposals drawn up following a three-week strike at the company over measures proposed by management to deal with the firm's financial crisis.
Bus Éireann workers are due to ballot on the proposals in early May.
However, while the Bus Éireann action was under way, the trade union Siptu organised ballots for its members in Iarnród Éireann and among school-bus drivers on whether they would be prepared to stage industrial action to support their CIÉ colleagues in Bus Éireann.
The school-bus drivers who were balloted are employed by Bus Éireann, but were not involved in the recent strike action.
Siptu organiser Paul Cullen said on Friday: "The ballot count has been completed and the majority of Siptu members in both Iarnród Éireann and those employed as school-bus drivers by Bus Éireann have voted to take industrial action in support of, and in sympathy with, their Bus Éireann colleagues.
“Our members in Iarnród Éireann voted by 73 per cent to 27per cent in favour of industrial action, while those employed as school-bus drivers by Bus Éireann voted by 72 per cent to 28 per cent in favour.
“Bus Éireann workers are commencing a ballot on a Labour Court recommendation concerning this dispute. Siptu representatives are currently holding meetings throughout the country with our members in Bus Éireann.
“The result of their ballot will be pivotal in deciding the course of this dispute,” he added.
Dublin Bus ballot
Earlier this month, Siptu’s Dublin Bus members voted by 67 per cent to 33 per cent in favour of taking industrial action in support of Bus Éireann workers.
The Irish Times reported earlier this week that Dublin Bus was likely to take legal action to prevent any future sympathy strikes by its staff in support of workers in Bus Éireann.
Management at Dublin Bus told members of the company’s board this week that its legal advice was that any such industrial action was likely to be unlawful.