People aged between 60 and 64 will be able to register online for a Covid-19 vaccination from Friday.
The HSE's vaccine portal has been open to people between the ages of 65 and 69 since last week.
To date, more than 143,000 have registered through the online system, Health Minister Stephen Donnelly said.
He added that vaccination of this group is already under way across the country.
From Friday it will be extended to people in the 60-64 age bracket.
Taoiseach Micheal Martin said: "The vaccine portal opens for everyone aged 60 to 64 tomorrow morning.
"The HSE are asking people aged 64 to register first on Friday for their Covid vaccine.
"We will then work our way down through those aged 63 to 60."
It comes as the National Immunisation Advisory Committee (NIAC) is set to meet on Thursday to discuss the European Medicines Agency conclusion that the benefits of the Johnson & Johnson vaccine outweigh the risks of unusual blood clotting as a very rare side-effect.
Almost 15,000 doses of the single-shot vaccine have arrived in Ireland but have yet to be administered.
Tanaiste Leo Varadkar has urged NIAC to allow the Johnson & Johnson jab to be given to people under the age of 60.
Last week the NIAC ruled that the under-60s should not be given the AstraZeneca vaccine due to a similar rare blood clotting issue.
Professor Sam McConkey said the Johnson & Johnson vaccine will be "very useful" in the rollout of the vaccination programme.
"I'm delighted the EMA has said 'Give it a licence'. It is licensed to sell here and I hope it is a substantial part of our national plan to get us out of Covid-19, and hopefully we can get a supply of it," he told Newstalk Breakfast.
"When I look at the small risk of clots... People have bandied around one in a million, but when we see the data from the US, that's sort of exactly where it's coming."
He said if the risk of severe illness or death from the virus is weighed up against the risk of severe side-effects, it differs depending on the person's age.
Prof McConkey said there is almost an "equal risk-benefit" from taking the vaccine for people aged around 20.
But he added that for those aged 25-30 there is "clearly a substantial benefit" from having the vaccine.
