An all-out strike by workers at Bus Éireann began in March
SIPTU members in Irish Rail and school bus drivers in Bus Éireann have voted in favour of taking industrial action.
They say this is in support of their colleagues in Bus Éireann.
SIPTU organiser, Paul Cullen, said: "The ballot count has been completed and the majority of SIPTU members in both Irish Rail 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."
Irish Rail members voted by 73% to 27% in favour of industrial action, while those employed as school bus drivers voted by 72% to 28%.
Bus Éireann workers are preparing to vote on a Labour Court recommendation aimed at solving the dispute.
While SIPTU representatives are holding meetings around the country with Bus Éireann members.
Mr Cullen says the result of their ballot will be "pivotal" in deciding the course of this dispute.
An all-out strike by workers at the embattled bus company over proposed cost-cutting measures began in March.
Wildcat strikes by Bus Éireann workers also brought Dublin Bus and Irish Rail services to a virtual standstill.
Workers refused to cross secondary picket lines, leading to nationwide cancellations.