Around 145,000 people awaiting a vaccine appointment are unable to go to their pharmacy to get one.
The Government today confirmed that people aged between 18 and 34 will be able get the Johnson & Johnson (J&J) vaccine in pharmacies from Monday.
It leaves people aged between 35 and 49 as the only adults who are unable to get the single-shot jab.
Many have been waiting weeks for their vaccine appointment and will now be waiting a minimum of 35 more days until they are fully vaccinated.
Meanwhile, an 18-year-old who gets the J&J jab on Monday will be fully vaccinated just 14 days later.
In a statement, the HSE said 35 to 49-year-olds were excluded because the Pfizer and Moderna vaccines are available to them.
It said around 926,000 people in the age cohort have registered through the vaccine portal.
Some 781,000 have received their first dose with over 375,000 having received two.
It means around 145,000 people are still awaiting an appointment and are ineligible for J&J.
While it is unclear how long they may have been waiting, the HSE said it aims to offer people appointments within three weeks of registering.
The Pfizer vaccine is administered in two doses around 28 days apart. You are classed as fully vaccinated seven days after your second jab – a minimum of 35 days after your first.
Moderna is also administered in two doses around 28 apart, with people classed as fully vaccinated 14 days after their second dose – a minimum of 32 days after the first.
J&J is a single jab with people classed as fully vaccinated 14 days later.