An alert has been sent to emergency departments and GPs in Dublin, Kildare and Wicklow after two confirmed cases of measles.
The Department of Public Health HSE-East has been notified of the cases in north Dublin city.
An Outbreak Control Team has been sent to investigate and control the spread of the illness.
The Health Service Executive (HSE) says the source of measles has not yet been identified.
It also says all people identified as contacts of the two measles cases are being notified and advised.
The HSE says: "The best protection against measles is to be fully vaccinated with two doses of the MMR (Measles-Mumps-Rubella) vaccine.
"If you have symptoms suggestive of measles you should stay at home, not go to school or work and phone your GP and explain that you may have measles."
People who have not been fully vaccinated with MMR vaccine, or have not had measles in the past, are at high risk if exposed.
Those most at risk are those who are not fully vaccinated - such as babies younger than 12 months, and those with weakened immune systems.
Dr Deirdre Mulholland, director of Public Health, said: "Measles can be a serious illness and is highly contagious. The best protection is to be fully vaccinated with 2 doses of MMR vaccine."
The time between exposure to measles and developing a rash is usually 14 days.
The HSE says people are infectious from four days before a rash starts until four days after.
- Symptoms include:
Red rash that starts on head and spread down the body
More information can be found here