44-year-olds can register for a Covid-19 vaccine from today.
The HSE's registration system will then open for 43-year-olds tomorrow, and will work down to 40-year olds on Sunday.
The health service says it expects appointments will start to be scheduled from the middle of this month.
Meanwhile, the Chief Medical Officer says the country will remain 'vulnerable' in June and July because a large amount of the population won't be vaccinated.
Dr Tony Holohan says no assurances can be given that the level of Covid-19 will remain similar to recent weeks.
In a letter to the Health Minister, he expressed concern about variants, particularly the Indian strain.
He says case numbers and the incidence rate remain relatively high.