No rail services will operate today, November 14, November 23 and December 8.
NBRU General-Secretary Dermot O'Leary said while the talks are "a potential step forward", he added: "It should be borne in mind that the intransigence displayed by Irish Rail management, led it should be said the CEO, together with tacit support from the Department of Transport, will make an overall resolution to this dispute more hard than it might otherwise have been".
It was later announced that drivers operating on Southern Railway have voted overwhelmingly to end its long running dispute over driver-only trains.
Billed as the "biggest disruption to passengers in decades" due to the number of operators affected, hundreds of services will be cancelled, replacement buses will be laid on and services that do run will be busier than normal.More news: Jay-Z Says Meek Mill's Prison Sentence is 'Unjust and Heavy Handed'
Train companies said they will try to run as many services as possible, but some routes into London will face disruption, and passengers have been warned to check before they travel.
The letter said: "I remain concerned that in the event of a train evacuation, derailment or incapacitated driver, the absence of a guard to assist could leave passengers at risk".
Four other railway lines - South Western Railway, Greater Anglia, Merseyrail and Arriva Rail North - will continue to take industrial action over the role of guards, with strikes taking place today and Thursday, reports ITV News.
"Similarly, anti-social behavior on trains could increase with the presence of a guard", he added.More news: Apple settled on Jersey for new tax shelter after Irish rule change
Irish Rail workers are seeking a pay rise of 3.75% without a provision for increased productivity.
A Transport Department spokesman said: 'The RMT is attempting to disrupt passengers as part of its political game.
Labor has already told the five rail operators that the party will halt any future plans to extend driver-only operations if it wins the next general election.
The announcement of strikes was made after a collapse of talks at the Workplace Relations Commission over pay.More news: Twins' Buxton, Dozier win Gold Gloves
"Driver-only operation has been operating safely for 32 years and now accounts for over a third of the United Kingdom rail network".
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