We report an outbreak of measles which started in April 2016 and which, by 13 June, has resulted in 22 confirmed and five probable measles cases occurring in four regions of Ireland. Genotype B3 was identified. We describe the identification, ongoing investigation and control measures being implemented. This outbreak occurs during a period of very low measles transmission in Ireland, with only one confirmed case (imported) notified in 2016 before this event.