Tallaght University Hospital has said it is operating what it called its Full Capacity Protocol due to high numbers attending its Emergency Department.
Members of the public have been asked to consider alternative care options before attending the hospital's Emergency Department as treatment is being given "in order of medical priority".
People with less urgent complaints have been told they will experience long wait times.
In a statement, the hospital said the Emergency Department is under pressure because of "high attendances and high rates of admissions", as well as "a shortage of beds".
The hospital said it is working to free beds for admitted patients, but said it is a challenge "with a high number of patients waiting home care packages or a place in a nursing home".
"There is a clear underlying requirement to develop additional onsite bed capacity given our growing and ageing population."
Tallaght University Hospital said it regrets any delays to patients and thanked the public for their co-operation.
