Some elective procedures that were due to be carried out at Mayo University Hospital today and tomorrow have been cancelled due to overcrowding.
In a statement, the HSE said more than 300 people attended the Emergency Department at MUH over the last two days.
As a result the hospital was "under significant pressure" with very long wait times for people who attend for routine and non-urgent treatment.
"The high number of people attending who need to be admitted for ongoing treatment means that there is pressure on bed availability throughout the hospital," the HSE said.
There were 20 patients on trolleys in the ED, with other patients on ward trolleys, at 8am this morning.
Some non-urgent elective procedures have been cancelled, with patients contacted directly if their procedure is affected.
People who do require emergency care are encouraged to still attend the hospital, but the HSE has asked "people to attend their GP or out of hours service in the first instance if at all possible".
