SIPTU yesterday said it will ballot transport workers on industrial action to support Bus Éireann staff
As the Bus Éireann strike continues, it is feared Dublin Bus and Irish Rail services could face disruption tomorrow.
A protest is being planned for outside Leinster House, to coincide with the Transport Minister's appearance at an Oireachtas Committee hearing.
Shane Ross will discuss the ongoing dispute at Bus Éireann - which revolves around the company's proposed cost-cutting measures - at the hearing.
SIPTU yesterday said its members in Dublin Bus and Irish Rail will be balloted on industrial action supporting their CIE colleagues.
Industry Correspondent with the Irish Times Martin Wall told Newstalk Breakfast any such action would take a number of weeks to decide.
However, he warns that passengers could be facing a more immediate problem tomorrow.
"Some Dublin Bus and Irish Rail staff will stop work for a period tomorrow to attend that protest," he explained.
"Any search action would be unofficial, and we'd have to wait and see how extensive it is - but there certainly does appear to be a prospect of work stoppages in the other CIE companies tomorrow, for a period at least."
Staff at Bus Éireann are taking part in the fifth day of all-out strike action today.
The strike got underway last Friday, with unions refusing to agree to the proposed changes to workers' pay & conditions as part of the company's cost-cutting plan.
Irish Rail says all train services are operating today, with no disruption expected as a result of the strike.
The board of Bus Éireann met yesterday to discuss the financial crisis at the company, with management later warning that losses at the company are now 'exacerbated'.
In a statement, the company said: "The Board could not sign off accounts for 2016, or pass a budget for 2017 in the absence of agreement with staff. This is a very serious matter as the Board must now formally advise CIÉ that this governance requirement will not be met.
"Without a plan which encompasses the necessary work practice changes to generate savings, it will not be possible to fund a voluntary redundancy scheme and faced with that scenario, the Board of Directors will have no option but to consider other measures to prevent the business becoming insolvent."
Dermot O'Leary of the NBRU described the statement as "bizarre and extraordinary".
He argued: "Compulsorily laying off staff will open a completely different and potentially uncontrollable dimension to this dispute, and will present severe difficulties in trying to maintain the already fragile industrial peace across other companies within the CIE Group."