SIPTU members at National Car Testing Centres (NCT) around the country have begun a series of one-day strikes in protest at a number of recent dismissals.
SIPTU spokesman Mr Christy Cullen said the company had dismissed ten members since 2000 and that in each case the Labour Courts Rights Commissioner declared the dismissals were unfair.
The union said 32 of the country's 43 NCT centres have been closed by the strike.
Mr Cullen said the union would continue to engage in one-day strikes at various centres "in order to impress on management that our members who will not accept such arbitrary unfair treatment at their hands".
However a spokeswoman for the NCT said only 24 centres were affected by the strike, and the company was not aware of any future strike action. She said that all centres would be operating as normal tomorrow.
Following discussions earlier this year, the company said it had introduced concealed cameras in the interests of safety and security and that SIPTU had agreed to the use of footage in rare cases of staff misconduct.
According to the company's spokeswoman, today's strike came as a surprise as they had been fully prepared to follow normal industrial relations procedures.
Customers affected by today's action were notified in advance and asked to make alternative appointments.