Strike action at driving test centres means up to 300 people who had tests booked this afternoon have had them cancelled.
Driving testers are taking the half day's action from 2 O'clock, in a dispute over outsourcing of testing work.
Their union IMPACT says management at the Road Safety Authority is planning to breach a Labour Court ruling, which set out a process for the recruitment of reserve testers.
Testers are warning the extent of any possible future action depends on the RSA's willingness to attend talks.
For its part, the RSA argues that in 2012 a total of 12,000 tests were not conducted because of tester sick leave.
IMPACT Assistant General Secretary Denis Rohan told Newstalk's Breakfast the RSA did not have the right to go against Labour Court recommendations in the way they outsourced the work:
