Two cases of measles in north Dublin city have been confirmed by the Department of Public Health and the Health Service Executive (HSE).
The HSE said an outbreak control team has been convened to investigate and control the spread of the potentially serious illness.
The source of measles has not yet been identified.
An alert has been sent to all emergency departments and GPs in Dublin, Kildare and Wicklow.
All people identified as contacts of the two measles cases are being notified and advised of the risk by public health.
Measles symptoms include high fever, cough, runny nose, red eyes , red rash that starts on head and spread down the body.
The rash consists of flat red or brown blotches, which can flow into each other.
The HSE advises anyone with symptoms suggestive of measles to stay at home, not go to school or work and contact their GP.
Measles is highly contagious and is spread easily. The time between exposure to measles and developing the rash is usually 14 days , but it can be more or less.
People are infectious from four days before the rash starts until four days after.
In a statement the HSE said people who have not been fully vaccinated with MMR vaccine or have not had measles in the past are at high risk of getting measles if exposed.
Those most at risk of catching measles are those who are not fully vaccinated with two doses of MMR vaccine such as babies and those with weakened immune systems.
More information on measles can be found here.