Two of the country's biggest motor insurers are refusing to cover cars 15 years or older.
Aviva and Allianz have blocked motorists from getting lower insurance prices, even if they hold a valid NCT.
The insurers claimed that older cars tended to be involved in more collisions, were used in fraud cases, had bald tyres and were poorly maintained.
The exclusion only applies to new customers, the com-panies said, adding that existing clients whose cars had passed the threshold would continue to be covered.
The move affects nearly 250,000 vehicles, or 13pc of the national fleet of 1.91 million cars.
The restrictions come amid a steady increase in motor insurance premiums, which rose 12pc last year.
Experts have warned motorists to expect further hikes this year of between 8pc and 15pc, due to the high cost of settling claims.
Aviva said the decision was taken based on risk assessments and applied even if a car was NCT compliant.
The move comes despite data from the Road Safety Authority (RSA) showing that vehicle condition is a contributory factor in only 0.2pc of all fatal and serious-injury collisions - or seven of more than 3,200 that happened in 2012.
The Consumers Association of Ireland said the move would restrict competition in the market and lead to higher costs for some motorists.
Restrictions on vehicle age are also applied by Liberty Insurance, which will not cover cars 20 years or older.
But not all companies are turning drivers away.
AXA said it had one of the widest acceptance criteria in the market and did not restrict vehicles but would seek a copy of the current NCT.
Other risk factors taken into account included the age of the motorist and their driving history.
Just over half of all fatal collisions in 2012 involved vehicles 10 years or older, but garda sources said it would be a "significant stretch" to link the condition of the car with fatal collisions, most of which were caused by driver error.