Three hospitals have introduced visitor restrictions due to an increase in patients presenting with flu-like symptoms.
Cork University Hospital is the latest to introduce visiting restrictions owing to the influenza outbreak.
Visitors to the hospital will only be permitted to paediatric wards, the ICU and to those who are critically ill.
Members of the public with flu symptoms are being asked not to come to the hospital at all.
Restrictions are also in place at St James's Hospital in Dublin and the Mercy University Hospital in Cork, with both saying they have taken the measure in the interest of patient safety.
Hospitals are advising anyone with flu symptoms to go to their GP rather than presenting at emergency departments.
The restrictions follow the number of people hospitalised with influenza more than doubling last week.
44 people were admitted to hospital with the virus during the week ending 1 February, compared with 18 reported cases the previous week.
Figures in the weekly Influenza Surveillance Report show there was also an increase in flu-related calls to GP out-of-hours service during this period.
However, the Health Protection Surveillance Centre said influenza activity is at moderate levels.
It says six people have died and 11 people have been admitted to critical care, due to influenza during this flu season.
St James's Hospital and the Mercy University Hospital say it is the first time they have ever taken such a measure because of flu.
St James's Hospital CEO and Medical Director Professor Patrick Plunkett has described the move as "unprecedented" and an "extreme measure" but said the hospital had no other option.
He said it was done to protect vulnerable patients because the flu virus could kill them.
Prof Plunkett said several wards in the Intensive Care Unit had been closed to planned admissions.
He said he was sure that emergency department overcrowding had contributed to the problem but was not the exclusive cause.
The decision will be reviewed tomorrow to see if restrictions need to be continued, he added.
The Mercy University Hospital has also restricted patient visit with the exception of the children's ward, critical care areas and some exceptional cases.
It says the restrictions will be continuously reviewed over the weekend.
Meanwhile, the Heath Service Executive said the number of people being hospitalised due to influenza is not high for the time of year.
It added there is some international evidence to suggest the strain of flu has drifted from the one that is being used in this year’s vaccine.
However, the HSE says it has not changed its recommendation that those at risk should get the vaccine, as it remains the most effective means of preventing infection.