Unions at Irish Rail are to ballot their members for industrial action.
Siptu's national rail committee is meeting this Thursday to discuss a ballot for strike action.
The union has referred its pay claim for all grades at Iarnród Éireann to the Workplace Relations Commission (WRC).
Siptu said it had informed management in writing that its "refusal to engage meaningfully on the pay claim" has resulted in the referral to the WRC.
Siptu organiser Paul Cullen said members were furious that their pay was behind industry norms.
He said: "Our members are completely vexed with the fact they have fallen behind a number of others within Iarnród Éireann and within industry in regard to pay.
"We haven't had the 6% we were due since 2008 and we feel we have contributed considerably to the company's financial position in the last number of years through cost-saving measures."
Meanwhile, the NBRU, which referred a number of issues to the WRC at the end of last month, is accusing Irish Rail of ignoring the industrial relations institutions of the state by refusing to deal with ongoing pay cuts, a pay claim and a 10-minute DART service.
In a statement, Irish Rail said there was a "credibility gap" in the unions' claims.
It said: "The NBRU claims to wish to engage on a range of issues, but has itself undertaken a series of walk-outs, unofficial actions and refusals to engage with the company.
"In recent weeks, the NBRU have:
- Walked out of ongoing discussions at the Workplace Relations Commission on the implementation of a 10-minute frequency DART schedule (meeting planned for 22nd March, NBRU withdrew on 18th March)
- Refused just yesterday evening to attend a meeting directly with Iarnród Éireann management this week to discuss issues surrounding DART drivers.
- Orchestrated unofficial action to prevent the in-cab familiarisation phase of training for new DART drivers.
The NBRU continues to ignore the fact there is an existing pay agreement with all trade unions in place across Iarnród Éireann which expires in October of this year. This agreement includes a temporary 25-month pay foregoing of basic pay ranging from 1.7% to 6.1%, which will be fully restored in October.
"In October 2015, the NBRU turned down an opportunity for increased earnings for DART drivers offered by management of up to 8% through a series of productivity measures, at a WRC-facilitated engagement.
"The company and trade unions – including NBRU – are currently engaged in a process under the auspices of the Labour Court to examine opportunities for productivity-relating earnings increases for all other grades.
"Iarnród Éireann believe the NBRU should – rather than planning ballots for industrial action - focus its efforts on engaging with all existing processes to ensure the company can deliver a better service to its customers, restore financial stability, and meet the aspiration for increased earnings amongst employees."