Thousands will be caught up in all-out rail strikes on the two weekends before Christmas unless staff get the same pay rise as Luas and bus drivers, the Herald can reveal.
Sources say the peak travel dates of December 16, 17, 23 and 24 will also be targeted - affecting shoppers and people travelling for the holidays.
Union chiefs threatened to ramp up the industrial action during the festive season and warned it could spill into the new year.
Yesterday saw the first of five 24-hour strikes that have already been announced for the coming weeks.
Taoiseach Leo Varadkar warned that workers will end up with a deal that "will probably be no different than if there'd been no strike at all".
"I think it's unfortunate that in the transport sector it seems it's impossible to come to an agreement without a few days of strike," he said.
Business groups warned that retailers would lose €150,000 a day over Christmas and accused unions of a cynical and deliberate ploy to target festive shoppers.
Up to 155,000 people - including 70,000 who use the Dart, 45,000 on commuter services and 40,000 who travel on InterCity routes - were hit during yesterday's stoppage.
But the impact was lessened because of the mid-term break, as some parents took time off work while schools shut.
Staff will mount another four 24-hour strikes - on November 7, 14 and 23 and December 8 - in pursuit of a 3.75pc a year pay rise.
General Secretary of the National Bus and Railworkers Union (NBRU), Dermot O'Leary, warned that the strikes could continue into 2018.
He said a "miserly" 1.7pc pay rise will not resolve the dispute and staff want a credible, no-strings-attached pay rise.
"If it takes five days and indeed more to prosecute and effect that, we will do that and unfortunately that would drag into Christmas and the new year," said Mr O'Leary.
Siptu's Reg Ennis also said the industrial action could escalate in the run up to Christmas unless there is a better pay offer.
However, Irish SME Association boss Neil McDonnell said it was very cynical to be attacking events like the World Cup play-off on November 14 and the traditional Christmas shopping day on December 8.
"This is just cut and paste stuff from the NBRU," he said. "We've heard it before in the bus strike and the Luas strike.
"This is really hurting retail jobs and employers right at the time when they would be expecting their best time of year."