School bus drivers are to be balloted on joining the strike over cost-cutting measures at Bus Éireann, which is now in its 11th day.
SIPTU has said the ballot will be concluded within three weeks and that, if sanctioned, the school bus drivers would join the industrial action after Easter.
SIPTU Transport Sector Organiser Willie Noone has said school bus drivers are being balloted because their jobs are on the line.
Speaking on RTÉ's Six One, Mr Noone said: "We must now take the action to bring them into this dispute because they don't want to stand idly by and wait until the very last minute to then start balloting."
"Quite clearly, Bus Éireann is the employer and if Bus Éireann doesn't exist anymore as an employer, the Bus Éireann school bus drivers will have no jobs.
"There's a direct link between their security of employment and the survival of Bus Éireann. It's not tenable for them to continue not being involved in this dispute in circumstances where their jobs are now on the line."
The development comes ahead of an expected announcement by the Workplace Relations Commission that it may intervene in the dispute in the coming days.
However, SIPTU - some of whose members are among those on strike at Bus Éireann - said that even if it does go into talks at the WRC, it will not call off its industrial action.
Dermot O'Leary, General Secretary of the National Bus and Rail Union has said that the union anticipates to be "asked to attend the talks in the next day or so."
He also said that NBRU members are "adamant that they will remain on the Picket lines pending the outcomes of such discussions."
Earlier, it confirmed that it received a solicitor's letter to the effect that the CIÉ Group intends to pursue the union for costs incurred as a result of the wildcat strike on Friday.
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Bus Éireann workers placed secondary pickets on Iarnród Éireann and Dublin Bus depots three days ago.
As a result of workers at those two companies refusing to pass the pickets, many transport services were suspended.
Mr O'Leary said the unofficial action should not have happened and appealed to members not to take part in any similar protest in the future.
Speaking on RTÉ's Today with Sean O'Rourke presented by Cormac Ó hEadhra, Mr O'Leary said that the NBRU had no prior knowledge of the action.
He said he would not take full responsibility for everyone who took part in the action, saying five unions had been involved. But he said he would take responsibility for NBRU members who had taken part.
The NBRU, he said, would robustly defend itself against any court action by the CIÉ Group.
Bus Éireann has said it regrets the ongoing disruption to the travelling public caused by the unions' strike action.
The company said it remains available for time-limited talks on efficiencies and changes to work practices.
It also said management and the board want to see a resolution, but are responsible for ensuring that the business can survive.
