SIPTU has said the latest work stoppage is "regrettable but inevitable due to management inaction"
The second of five proposed train strikes is taking place today, with no train services operating across the country.
The industrial action is being taken by Irish Rail workers as part of a dispute over pay.
The transport company has said it has offered a 1.75% increase for one year to employees.
However, workers are calling for a pay increase of 3.75% a year for the next three years, with unions saying staff have not been awarded a pay rise since 2008.
An estimated 150,000 passengers have had their train journeys cancelled today across DART, Commuter and Intercity services, with refunds being offered to all affected customers.
Luas into town already noticeably busier as commuters plan around the rail strike pic.twitter.com/aDISafhPkR— Sean Defoe (@SeanDefoe) November 7, 2017
Further strikes are already scheduled for Tuesday 14th November, Thursday 23rd November, and Friday 8th December - with next week's strike set to affect football fans travelling to the Aviva Stadium in Dublin for the Ireland v Denmark World Cup play-off.
Irish Rail's Barry Kenny is urging the trade unions to either go back to the Workplace Relations Commission or refer the outstanding issues to the Labour Court instead of "threatening further days of industrial action".
Unions, meanwhile, have described the latest work stoppage as "regrettable but inevitable".
SIPTU organiser Greg Ennis argued: “Our members do not wish to be returning to the picket lines [...] but they have been left with no alternative.
"They remain resolute and strong in their belief that it is only by conducting industrial action that their concerns about the future of the company and the failure of staff to receive a pay rise in 10 years will be adequately highlighted.”
He called on Transport Minister Shane Ross to intervene, and warned that a further escalation of industrial action in the lead-up to Christmas is an "unfortunate but very real possibility".
NBRU general secretary Dermot O'Leary, however, told The Pat Kenny Show that he is not calling for Minister Ross to intervene directly - saying he's "sick and tired of listening to people talking for ministers to intervene".