Anyone aged 18 to 24 will be able to register for an AstraZeneca vaccine from Monday, the HSE has confirmed.
They'll also be given a target date for when registration for mRNA vaccines will open, so they can make a decision on whether they'd prefer to wait.
It comes as all 25-29 year olds can now register for either a Pfizer or Moderna mRNA vaccine.
Those aged 18-30 can already receive a Janssen vaccine from a pharmacy, although supplies of the one-shot jab remain limited.
Health officials continue to recommend anyone under 50 should get one of the mRNA vaccines, although the vaccine choice has been expanded amid concern over the Delta variant.
On The Pat Kenny Show, the HSE's vaccine rollout lead Damien McCallion said there'll be more progress on Monday.
He explained: "We’ll open up the opt-in for the vaccination centres for our 18 to 24-year-olds. On Monday, we’ll also set a date for when people in that age will be able to get an mRNA vaccine - we’d anticipate that being within a number of weeks.
“Anyone who’d take up the opportunity to register for that [AstraZeneca] opt-in… would get their second dose within four weeks [of the first].
“We’ll be clear with people: the preferred vaccine, as expressed by NIAC, is the mRNA vaccine. We’ll also be clear with people what the target date would be if they waited for an mRNA vaccine.”
Meanwhile, appointments for those who register today will start being sent out "almost immediately" - with some appointments likely to be scheduled from as soon as early next week.
Mr McCallion cautioned there will be variations around the country in terms of the rollout, but officials hope to get everyone in the 25-29 age group their first dose within three weeks.
He said Ireland now has a stable weekly supply of vaccines, particularly from Pfizer, while talks are ongoing with countries such as Romania about potential extra supply.
He said it will be up to NIAC about whether there'll be a vaccine 'booster' programme, but the HSE is already planning and considering what such a programme would look like.