The Health Service Executive (HSE) says it has confirmed 17 cases of measles.
There are 12 confirmed cases in Dublin, with most cases in the north of the city, and a smaller outbreak of measles in Meath with five confirmed cases.
It says both children and adults have been affected.
Dr Deirdre Mulholland is director of public health for HSE East. She says measles can be a serious illness and is highly contagious.
"The best protection is to be fully vaccinated with two doses of MMR vaccine."
The HSE advises that everyone should be aware of the possible risk of spread of measles whenever groups of people gather, and particularly when attending healthcare services like GPs or emergency departments.
A Measles Outbreak Control Team is investigating the cases, the HSE adds.
It has also repeated the following advice on measures to control further spread of the illness:
- All children should get the MMR vaccine when they are aged 12 months. If any child aged over 12 months has missed this vaccine they should get it now from their GP
- All children should get a second dose of MMR vaccine when they are four to five-years-old or in junior infants at school. If any child in senior infants or older has missed this vaccine they should get it now from their GP
- Adults under 40 years who have not had measles or have not received two doses of MMR vaccine should contact their GP to get the MMR vaccine
Those who think they may have measles are advised to stay home, phone your GP and stop visitors coming to visit you to prevent the spread of measles.
Pregnant women who have been exposed to measles should seek medical advice as soon as possible.
- Measles symptoms include:
Red rash that starts on head and spread down the body - this normally starts a few days after onset of illness
Vomiting, diarrhoea and tummy pain may also happen