Vaccination of children aged 12-15 will begin 'within days' of registration opening this Thursday, the HSE says.
Officials say they've "held back as many places as possible" this weekend so teenagers can get COVID-19 vaccines quickly.
It means people in the age group could get their first doses as soon as Saturday or Sunday.
The vaccine portal opens for 12-15-year-olds on Thursday.
Teenagers will need parental consent in order to get vaccinated, and they will receive either the Pfizer or Moderna vaccines.
Most vaccination of the age group will take place in the country's mass vaccination centres, although some GPs and pharmacies will also be able to administer jabs.
Damien McCallion, the HSE's national lead for the COVID-19 vaccination programme, told Newstalk Breakfast vaccination will begin almost immediately - and the HSE is "optimistic" there'll be high take-up.
He said: “The registration system will open on Thursday, and we would anticipate being able to start vaccination within a couple of days of registration.
“What we’ve done this weekend is try to hold back clinic places.
"We’re going to try to allocate as many slots as possible for those parents who register their children on Thursday, Friday and into the weekend."
There won't be walk-in clinics for teenagers at first, with Mr McCallion saying they'll look again to see if they're needed later in the rollout.
He noted the HSE is also looking at evening clinics with extended opening hours, due to the logistical challenges of parents bringing children to get vaccinated during weekday working hours.
'Nobody will be left behind'
Vaccine registration can all be done online, although the parental consent process can also be completed at the vaccination centres.
Mr McCallion said officials are “very conscious” of parents’ concerns and questions around vaccinating children.
However, he said people should take comfort in NIAC’s ‘methodological’ approach in giving the green light for vaccinating 12-15-year-olds.
He recommends that parents and children look at the HSE website and the medical advice available there before making a decision.
With the HSE expecting that most adults will be fully vaccinated by the end of this month, Mr McCallion said vaccination of everyone aged 12 or older should be complete by mid-late September.
He stressed that "nobody will be left behind", and people will still have ways to get vaccinated even after the mass vaccination programme winds down.
Meanwhile, the HSE is now starting to look at what a booster vaccine campaign will look like in the autumn and winter, following initial advice from NIAC on the issue.
Mr McCallion said such a programme is likely to get underway from October, with officials now looking at which groups would be prioritised for a booster shot.