Almost a fifth of all crime reported to the Gardaí is still not recorded on its own system, official analysis has found.
And the Garda Siochana's success rate in solving crimes is probably 10% lower than actually claimed, the Central Statistics Office (CSO) has warned.
Despite widely-publicised concerns being flagged several years ago on reported crime figures, the CSO has again cast doubt over official numbers.
The latest caution was sounded as figures were released claiming the number of burglaries had plunged 26%, thefts were down 12% and sexual offences were up 13% in the year to June.
But the CSO said the country could not have "full confidence" in the figures until outstanding issues with how the force recorded crime were resolved.
Tim Linehan, of the CSO, said the scale of crime still not being recorded on the official Garda database Pulse had only fallen "very slightly" over the last year.
"Around 17% of crime reported to An Garda Siochana in 2015 was not recorded on Pulse," he said.
Last year, it was reported that 20% - or one in five - of all reported crimes never made it onto the system.
In a follow-up inquiry into last year's official records, the State agency again portrayed confusion and huge differences in how crimes were recorded in Garda stations across the country.
Some stations used technology and paper, some used only one or the other.
The CSO said where stations did not keep matching paper records it was "ultimately impossible to measure" how much crime was not being properly recorded.
Doubts were further cast on the classification of around 5% of major crimes.
Some 3% of crimes - including burglaries and thefts - were found to be recorded in the wrong category. Another 2% of records did not have enough information to tell whether or not they were correctly classified.
The CSO said 18% of crimes marked as detected - or solved - had no charge or summons sheet attached to prove it.
Removing these would reduce the Garda Siochana's overall claimed crime detection rate by 10%, it said.
"The CSO is continuing to work with An Garda Siochana to improve the reliability of the data and will continue this analysis at regular intervals to monitor data quality," Mr Linehan said.
Three years ago, former Garda chief Martin Callinan dismissed as "simply not true" allegations by the Garda Representative Association (GRA) that some crimes were not being recorded while official figures were easily massaged.
A year later,the CSO launched an investigation after the Garda Inspectorate exposed massive errors on the Pulse system, including poor classification of incidents and under-reporting, throwing doubt over the country's true crime rates.