Update 3.30pm: It was revealed today that Unite has amassed a €40 million strike fund to support members during industrial disputes.
The news comes as workers in Bus Éireann are on a fifth day of industrial action in protest at the unilateral imposition of a cuts package.
"As our members spend another day on the picket lines, they need to know that they have the full support of 1.4 million Unite members across Britain and Ireland,"said Unite general secretary Len McCluskey.
"Over a number of years, Unite has amassed a €40 million strike fund to support members during industrial disputes. I am sending a clear message to employers: when our members are in dispute, they not only have right on their side – they also have financial might on their side.
"I hope that the Irish Government will see sense, start treating Bus Éireann as the vital strategic asset it is, and invest in its services accordingly. But our members will not be bullied to the table by threats.
"They are fighting not only for their own futures, but for the right of communities throughout Ireland to a decent public transport system that provides a quality service and quality jobs. They have my full support in that battle - and the backing of a €40 million strike fund."
Update 1.30pm: Irish Rail has said they will not be affected by strike action tomorrow.
Earlier we reported that all three transport bodies, Irish Rail, Bus Eireann and Dublin Bus will be taking part in a group demonstration outside Leinster House.
However, Irish Rail has contacted our publication via Twitter to confirm there will be no disruption to their services tomorrow.
We have no indication of any disruption. We will update customers if this changes.— Iarnród Éireann (@IrishRail) March 28, 2017
Update: 1pm: The head of the National Bus and Rail Union says he may not be able to stop his members at Irish Rail and Dublin Bus from going on a 'wild cat' strike.
Bus Éireann workers are into day five of an all-out, indefinite strike.
SIPTU is to ballot its members in Irish Rail and Dublin Bus, following a threat yesterday by Bus Éireann that compulsory redundancies may be unavoidable.
But Dermot O'Leary from the NBRU won't be balloting his members on legal advice.
And he says the longer the strike continues, the more likely it is that others in the sector will join the picket lines: "My members in Irish Rail and Dublin Bus are extremely angry that we have now gone into five days.
"I'm trying to hold the line with those members, now five days, six days, seven days, I don’t know how long I will be able to hold that.
"But it would be remiss of me not to explain to the public, at least, and the politicians that are listening to this, that I won't be able to hold that line forever.
"There is a moral obligation on me here, and I will come under pressure from that moral obligation alone."
Latest 12pm: Siptu has confirmed it is legal for staff in Irish Rail and Dublin Bus to go on sympathy strike with Bus Éireann staff.
All three CIE firms will be disrupted tomorrow as workers join a group demonstration outside Leinster House.
Protests could become more permanent as union colleagues across the industry are balloted on the possibility of properly joining the dispute and cancelling services.
Siptu's Willie Noone says all transport workers have a right to take part.
We wouldn't have gone down this road with it.
We're getting an unusual reaction to an unusual situation," said Noone.
"They're not willing to stand by and let Bus Éireann just be sold off to the lowest bidder."
Update: 9.22am: Siptu has warned there is a level of anger in the transport sector that has not been seen in decades.
The trade union says Irish Rail and Dublin Bus workers are demanding to show their support for their Bus Eireann colleagues whose strike has entered its fifth day.
Staff from all three CIE firms will join together for a demonstration outside Leinster House tomorrow while the Transport Minister discusses the issue inside.
Siptu's Willie Noone says it reflects a feeling of solidarity among workers.
Earlier: The Bus Eireann strike has entered its fifth day, amid fears it could spread to Dublin Bus and Irish Rail.
Bus Eireann management is suggesting it may be forced to make compulsory redundancies if unions do not agree to cost cutting measures.
The NBRU says "all bets are off" if the company tries to axe jobs in the troubled company.
SIPTU is balloting its members at Dublin Bus and Irish Rail on taking industrial action in support of Bus Eireann workers.
Dermot O'Leary of the NBRU says he is also exploring that option: "If I'm honest, I'm unsure of the legalities around such a ballot in the first place.
"I am seeking legal advice at the moment from our legal people to see if we are able to even do that."