The HSE says vaccinating people in their 30s against COVID-19 will take longer than previous age groups.
People aged 35 to 39 will be able to register for a COVID-19 vaccine within days, starting with 39-year-olds from Sunday.
Almost 3.5 million doses of the vaccine have so far been issued, with 31 per cent of the adult population fully vaccinated.
Over 60% have received at least one dose of a vaccine.
HSE CEO Paul Reid says there are two reasons it will take longer to vaccinate those in their 30s.
He said: "To flag upfront: this will be a slower age group to move through based on volume, but particularly based on the high-level of dose two administrations we will have to do.
"We also move to two supply lines in July - which is Moderna and Pfizer."
A majority of Ireland's remaining AstraZeneca vaccines are now committed to second doses, with authorities moving to speed up those second doses for people who've only received one dose so far.
Officials here have also recommended that both the AstraZeneca and Johnson & Johnson jabs only be given to older adults.
It means younger age groups will be vaccinated with the two mRNA vaccines - Pfizer and Moderna.