SIPTU members in Irish Rail and school bus drivers with Bus Éireann have backed industrial action in support of their colleagues in Bus Éireann in a ballot today.
However, the strike mandate will only be implemented if Bus Éireann staff reject a Labour Court recommendation which is currently being put out to ballot.
The support ballot of school bus drivers and Irish Rail staff was commenced during the three-week strike by Bus Éireann staff in a dispute over cuts to pay and conditions - though school bus drivers were not included in the industrial action at that time.
SIPTU Organiser, Paul Cullen, said: "Our members in Irish Rail voted by 73% to 27% in favour of industrial action while those employed as school bus drivers by Bus Éireann voted by 72% to 28% in favour."
SIPTU members at Dublin Bus have already voted to take action in support of their colleagues if required.
However, sources have suggested that if they proceeded to do so, Dublin Bus management could challenge the legality of any such moves.
"SIPTU representatives will now liaise with our colleagues involved in the Bus Éireann dispute to discuss when to serve the necessary notice of industrial action on management," Mr Cullen added.
The Bus Éireann strike was suspended to allow the five unions at the company to ballot on a Labour Court recommendation aimed at resolving the dispute, with a result expected in around a fortnight.
The proposals would see significant changes for staff, including over 200 redundancies, the amalgamation of certain premium payments into a higher core basic pay rate, and new work practices.
Bus Éireann management has argued that the company, which lost €9.4m last year, could face insolvency next month without the proposed cuts.
If the recommendation is accepted by staff, the ballots at Dublin Bus and Irish Rail may become irrelevant.
However, if staff reject the proposals, there could be transport chaos with a dispute encompassing all three CIE subsidiaries at the same time.