Update 1.45pm: Some homicides were registered as non-crimes on an outdated and inadequate Garda computer database, the force has said.
Twelve deaths out of 41 were reclassified on the 20-year-old Pulse IT system after mis-recording was discovered, an assistant commissioner said.
It followed checks with the pathologist's office, which investigates medical causes of death.
Garda analyst Gurchand Singh said: "When we looked on Pulse it was not there as a homicide, it was often recorded in a lesser group, a non-crime group such as sudden death, or it was recorded as an assault.
"It was not corresponding, the report from the State Pathologist was not corresponding to what was in Pulse."
Records on Pulse had not been updated but that did not mean cases had been improperly investigated, a senior officer told the Oireachtas Joint Committee on Justice and Equality.
Assistant Commissioner Michael Finn said: "The IT system is 20 years old and not fit for purpose.
"That is at the root of a lot of issues we are discussing.
"Technology has moved on massively in 20 years."
He said promised investment from Government was needed but it would take some time to overhaul the system.
A review is examining all homicide cases including fatal road traffic collisions from 2003 to last year.
Mr Singh, head of the Garda's analysis service, said it would take five or six months to identify "problematic" incidents.
Mr Finn said 12 out of 41 cases had been reclassified, families of the victims notified and pointed towards further support.
He said: "In our IT systems it was not recorded correctly.
"However, in the investigation files you discovered they were investigated.
"In terms of the amount of work we have done and what we are doing, it would be unfair to say we are not taking this seriously or treating it as a minor matter."
The committee was told that more extensive reviews were launched following the discovery of concerns with Pulse following an internal investigation.
Independent TD Clare Daly said the mis-recording had been traumatic for victims' families.
Update 12.32pm: Head of Garda Analysis says team did not sign off on final misclassification of homicide figures report
The head of Garda Analysis says his team did not get to sign off on the final report into the misclassification of homicide figures.
Senior members of the force are appearing before the Oireachtas Justice committee after a review found that 12 deaths should be reclassified as homicides.
The Policing Authority is to conduct an independent review, despite the Gardaí saying the cases were properly investigated.
Head of Analysis Gurchand Singh told the Committee he raised concerns about the data and not seeing the final report.
He said: "When we raised it in November, part of the purpose was to say we had concerns.
"What we wanted to do was to have this review group and see if the concerns were real.
"For whatever reason we didn't get a chance to sign off on that final report and when we did review that final report, there was elements we couldn't agree with."
Earlier: Gardaí will be asked to clarify homicide figures today
Senior members of Garda management will appear before an Oireachtas committee today over the classification of homicide figures.
A review last year of 524 deaths saw 12 re-classified as homicides.
Gardaí say the cases were properly investigated - but the Policing Authority has expressed concerns and is to conduct an independent review.
Fianna Fáil TD Jack Chambers is a member of the Oireachtas justice committee, he says it is important that the Gardaí have full space and time to give full information on the back ground of this.
He said: "It will also bring clarity to some of the concerns that have been expressed by the policing authority.
"I look forward to them giving the detail and setting a pathway around the independent process that is going to be conducted."
- Digital Desk