The Policing Authority has found more than half of the commitments made by gardaí in the 2018 Policing Plan were not achieved.
In its full-year assessment of how the force performed, it said the detection of crime was a particular area of concern.
The authority said that "detections reflect the organisation's capacity to solve crime" and are "imperative in maintaining public confidence".
It added that detections "have been falling since at least 2010".
Areas of progress include that targets for identifying suspects and victims of online child sexual abuse were exceeded, the number of burglaries were down and the reporting of sexual offences increased.
Significant progress was also made in targetting organised crime groups, including an increase in the number of firearms seized by gardaí.
But while acknowledging this progress, the report found that almost all detection targets were missed.
In the area of roads policing, 2018 saw the lowest number of deaths on our roads at 149.
The report found that while 150 new members of An Garda Síochána had been appointed to road policing, speed detections were down by 20%.
There were improvements in recruitment with 798 new probationary Garda members in 2018.
Substantial progress was also made with implementing key ICT projects.
However, recruitment of staff and reserves "fell significantly short", as did redeployment of staff to the frontline.
The Policing Authority said that resources and capacity continue to be a major barrier in achieving objectives.
There was a "substantial improvement" in reducing the backlog in forensic computer examinations, and the authority said resourcing was a serious concern for the effective operation of cyber crime functions.
Overall, An Garda Síochána met just 53 of the 116 commitments measured.
It was the authority's view that many of the issues in 2018 "were not just predictable, but predicted" and that the organisation itself was aware from the start of the year that the plan was "extremely unlikely to be fully delivered".
Public trust in the force remains high, with nearly 90% of people surveyed having a mid-to high level of trust.
The perception that the organisation is well managed increased overall by 1% - to 39%.
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