All 25 to 29-year-olds can register for a Pfizer or Moderna COVID-19 vaccine from today.
The HSE's registration portal is now open to everyone in the age group.
Unlike previous expansions of the vaccine programme, registration is open to everybody aged 25-29 immediately and is not happening on a year-by-year basis.
While health officials continue to recommend mRNA vaccines (Pfizer or Moderna) for those aged under 50, everyone aged 18-30 can still register with a pharmacy for a one-shot Janssen vaccine.
It comes as more than five million doses of COVID-19 vaccines have now been administered in the Republic.
Over 60% of adults are now fully vaccinated, while 75% of adults have had at least one dose.
Chair of the vaccine taskforce, Professor Brian MacCraith, said the five million dose milestone is a very significant day in the vaccination programme.
He said: "Over the past number of weeks, we've been hitting typically around 340-350,000 doses a week - [frequently] well over 50,000 per day.
"It has really been ramping up again and again over recent weeks.
"We've still a good deal of a distance to go - we want to get a very large fraction of adults vaccinated."
He said work is currently ongoing to vaccinate everyone aged 16 or older, while NIAC will soon make a decision around whether to allow the vaccination of teenagers aged 12-15.
Meanwhile, a protest took place in the Phoenix Park yesterday evening, with several hundred people demonstrating against plans to reopen indoor dining for only vaccinated and immune people.
A crowd of several hundred people gathered at the gates of Áras an Uachtaráin, following a similar demonstration outside the Convention Centre (where the Dáil was sitting) earlier this week.
Demonstrators were urging President Higgins not to sign the indoor dining legislation, while some carried signs and placards with anti-vaccination messages.
Others in the crowd held Irish flags, American flags, and flags displaying religious iconography.
There were no reports of any arrests, with gardaí saying they monitored the protest.
The legislation around indoor dining will be debated in the Seanad later today, after passing all stages in the Oireachtas earlier this week.
Tánaiste Leo Varadkar yesterday said it's now planned that indoor hospitality will be able to reopen from July 26th under the proposed legislation.