Two cases of measles have been identified in north Dublin, the HSE has said.
It said an outbreak control team had been convened to "investigate and control the spread of this potentially serious illness".
The source of the two cases has not been identified, said the Department of Public Health HSE East.
An alert has been sent to all Emergency Departments and GPs in Dublin, Kildare and Wicklow and people identified as having been in contact with the two measles cases have been advised of the risk.
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 two doses of MMR vaccine."