Train drivers at Irish Rail have rejected a pay rise in return for increased productivity.
The Labour Court had recommended a 1.15 percent pay hike to allow for driver training which would lead to more frequent services.
The company has said it "regrets" the decision by driver trade unions SIPTU and NBRU to reject the Labour Court recommendation on driver training and productivity issues.
Irish Rail says the "non-cooperation" has resulted in repeated delays to the introduction of an increased 10-minute DART frequency, and prevented expansion in other services including off-peak commuter services.
SIPTU Organiser, Paul Cullen, said "the ball is now firmly in management’s court and their actions over the coming days will determine whether or not a fair and speedy resolution can be found."
"However, if management attempts to provoke a dispute, our members will not be found wanting in delivering a fair, swift and justifiable response", he warned.