Staff in Beaumont Hospital have the worst record in Dublin hospitals for washing their hands - a key infection-control measure to prevent patients getting potentially life-threatening illnesses.
Although rates have generally improved in recent years, too many doctors, nurses and other staff are still not following rules and are threatening patient safety.
New figures showed that one-in-five staff in Beaumont was still not following the basic safety standard.
In response, Beaumont hospital said it was committed to ensuring the highest level of compliance with hand hygiene.
"Compliance rates fluctuate over time and between different clinical areas and a more comprehensive hand-hygiene compliance audit undertaken in Q4 2015 found average compliance across all wards in Beaumont was 92.7pc," it said.
"We continue to focus on hand hygiene and there are a wide range of measures to improve compliance in place ."
Hand-washing rates in the Mater Hospital are at 89pc, which leaves one-in-ten staff putting sick patients in danger of getting another illness.
Both the main children's hospitals of Crumlin and Temple St have rates of around 92pc.
The private Blackrock Clinic only managed a rate of 81.4pc when inspectors from the Health Protection Surveillance Centre conducted their probe.
Nationally, the HSE said hand-washing compliance rates in Irish hospitals now stand at 89.2pc - marginally below the HSE target of 90pc.
Microbiologist Dr Robert Cunney said: "Hand hygiene is the single most effective measure that healthcare workers, patients and the public can undertake to prevent healthcare associated infection."
"The progress to date is very welcome, but improving it further is a priority for the HSE."
He said good hand-hygiene compliance is also important for patients and visitors.
Hand cleaning should be performed after coughing or sneezing, after going to the toilet and before mealtimes.
More than one-in-20 patients will pick up an infection in an Irish hospital, including superbugs such as MRSA.
A range of other illnesses can also be spread by contaminated hands.
These include pneumonia related infections, urinary tract infections and the winter vomiting bug.