Detection rates for sex crimes are down by almost a fifth, a Garda watchdog has been told.
The Policing Authority was told the number of prosecutions this year is not matching the marginal increase in the number of cases being reported.
There was a 7% increase in recorded sex crimes this year to the end of August.
The figures were revealed at a public hearing in Dublin Castle despite the Central Statistics Office (CSO) having refused to publish crime statistics for 2017 after the Garda admitted errors in its homicide records.
Judith Gillespie, former Police Service of Northern Ireland deputy chief constable, told the hearing that detections - which include a suspect being charged or summonsed - are down 19%.
"In those most serious contact, violent crimes and sexual crimes, actually the detection rates have quite seriously deteriorated," she said.
"What we do know is that recorded sex crime has gone up, it's marginal, and that the detection rate has gone down considerably."
John Twomey, Deputy Commissioner, said there is a "time lag" in sex crime investigations and that they must be treated sensitively.
But he told the hearing: "It's something that we are looking at.
"I'm confident that the detection rate, come year end and into the future will be of similar than previous years. I don't see this as being anything other than investigative issues.
"It's vitally important that we understand what we are talking about. Is there a key driver behind that reduction - we are not seeing anything that's specific.
"There's nothing that's causing us concern that people aren't doing one thing or another."
Gardai record a crime as being "detected" when a summons is served on someone or a suspect is charged, or when an adult cautions or the youth diversion programme is applied.
The Policing Authority was told that "detections" are also recorded even when a victim withdraws a statement or does not turn up in court for a prosecution to proceed.
Senior gardai at the hearing could not confirm how many of these instances they have recorded.
Dr Gurchand Singh, head of the Garda's analysis service, told the hearing that research by the CSO has found sexual offences have the highest rate of "fall off" where victims do not pursue a prosecution.
The Policing Authority was also told that the classification and re-classification of crimes will solely be administered at the Garda Information Services Centre in Castlebar, Co Mayo from next January or February.
In June the force admitted they understated the number of homicides by 89 over the past 14 years - a rate of more than six a year.
A final report on the issue has been submitted to the Policing Authority.
Other crime records for this year showed that assaults linked to the night-time economy could be up 15% if they continue at the current rate.