The Health Service Executive is writing letters to more than 5,000 people, inviting them to take part in a study to measure exposure to Covid-19.
Those who take part will be asked to complete a questionnaire and provide a blood sample. The letters are being sent to people in Dublin and Sligo to invite them to take part in the study.
It will measure exposure to Covid-19 in the population using an antibody blood test in counties with some of the highest and lowest levels of infection.
The HSE said that using a random sample of participants from both locations will make it possible to provide an overall national estimate of infection in the population.