Irish Rail says many of their drivers aren't crossing picket lines at stations they share with Bus Eireann.
It's led to widespread delays to train services, on top of the travel disruption already caused by the strike.
Barry Kenny from Irish Rail says the picket lines have been moved to locations that block access for their train drivers.
"In previous situations where one of those organisations has been involved in industrial action, the other has been able to access their own workplace ... that isn't the case today".
The below routes have all been cancelled:
Other routes are running a limited service. You can see the full list on IrishRail.ie.
Buses stopped at midnight after drivers refused to work under the threat of pay cuts.
Reps for the firm say they're extremely disappointed by the industrial action which will cause a major inconvenience to customers and "exacerbate the perilous financial situation at the company."
They've previously said the company faces going bust by May if it doesn't implement a €12m cost cutting plan, which includes job losses.
Unions had consistently promised that if the company pushed ahead with the controversial measures without their agreement, up to 2,400 staff would go on an all out strike.
Transport Minister Shane Ross says he's deeply concerned by the impact the strike will have on the travelling public.
"In the last week, both management and trade unions have highlighted their willingness to discuss improvements in the company’s operations and efficiency.
It is now time for both sides to show this willingness can result in an acceptable and fair agreement that safeguards the company’s future and all of those who rely upon it."