People who are immunocompromised will get their third dose of a COVID-19 vaccine from next week.
Healthcare professionals will identify patients who need the extra dose.
Patients will then be contacted from Wednesday, with appointments starting next Friday.
It will be given at least two months after their last dose of a vaccine.
However, the HSE claims this is not the beginning of their general booster campaign.
They say people who have a compromised immune system - such as certain cancer patients or people on certain medication - require three shots to reach the required level of immune response.
These doses are described as a "third primary dose", and will use either the Pfizer or Moderna vaccines.
In contrast, most people need two shots (or one shot of the Johnson & Johnson vaccine) to reach the required level of immune resonse.
Booster doses, meanwhile, are due to be rolled out to everyone aged 80 or older, as well as care home residents aged over 65.
Those doses are intended to combat potential waning immunity in people in the months after their initial vaccination.
That campaign is expected to get underway in the coming months, and those doses will be given at least six months after the second dose.
EU regulators say they're currently examining data from Pfizer on whether everyone aged 16 or older should receive a booster dose.
The WHO is continuing to urge wealthy countries to hold off on large-scale booster campaigns until more vaccines reach poorer countries.