Last ditch talks to avoid the walk out collapsed without resolution at the Workplace Relations Commission overnight.
That means there'll be no services until 9am – a handful of services that started before 6 will continue as normal.
The dispute centres on pay and productivity, but an 8 per cent pay rise offered by Iarnrod Eireann management was reportedly turned down by workers.
SIPTU's Paul Cullen doesn't foresee an end to the problem any time soon.
Online customers who're booked to travel during the strike can use their train tickets any time after it finishes today or tomorrow.
They can also apply for a refund on their credit card.
Dublin Bus meanwhile, expects services to be busier than normal today.
But spokesperson Carol Donoghue says there won't be any extra buses laid on to meet the increase in demand.
