Talks at the Labour Relations Commission aimed at averting a threatened one-day strike by Bus Éireann school-bus drivers were continuing last night.
More than 14,000 school children could be affected by the dispute if it goes ahead.
The row concerns pay levels for part-time school bus drivers.Siptu is seeking parity with full-time Bus Éireann drivers.
The National Bus and Rail Union wants a "substantial increase" for its part-time drivers.
Siptu said that the drivers earn €11.66 an hour and had no overtime rates, pension, sick pay or other benefits.
"They are amongst the lowest-paid workers in the public transport or education sectors. For example, school cleaners earn €13.99 an hour, school caretakers €14.38 and other Bus Éireann drivers have a pay scale running from €13.15 to €15.23 an hour", it said.
Siptu national industrial secretary Michael Halpenny said the rates in no way reflected the professionalism and demands of the job. He said that these had increased significantly over time, especially where safety and disciplinary issues were concerned.
Some 550 drivers providing school bus services around the country are employed by Bus Éireann.
The firm has argued that the Siptu claims have been rejected by the Labour Court and a rights commissioner.