Four in 10 buses, including those used for school transport, failed a roadworthiness test in the past year.
Of 1,171 roadside inspections carried out between July 2018 and June 2019, 456 (39pc) failed for a reason other than a "minor issue".
The figures were provided by the Road Safety Authority (RSA) to Fianna Fáil education spokesperson Thomas Byrne, who asked a parliamentary question about safety inspections on school buses.
RSA chief executive Moyagh Murdock said the inspection data related to buses generally rather than specifically buses involved in school transport, and provided no further breakdown.
She said that most bus operators took their responsibilities seriously in respect of maintenance conditions "but there is no room for complacency".
Ms Murdock added that they were working closely with gardaí to improve bus operator compliance and they would be making use of new powers for inspectors to "prohibit, detain, immobilise and seize dangerously defective vehicles".
Mr Byrne said the school transport fleet should be the safest in the country, but it was "clear from the overall figures that the quality remained a serious issue given that many of the buses inspected by the RSA operated in the school transport fleet".