The National Immunisation Advisory Committee (NIAC) will recommend to Government that the AstraZeneca and Janssen (Johnson & Johnson) vaccines can be administered to 18 to 40-year-olds, RTÉ News understands.
NIAC has also recommended that the AstraZeneca vaccine dose gap be reduced to four weeks, Government sources have told RTÉ News.
It is expected that this would allow many more people in their 60s to be vaccinated faster.
The current gap between dose one and two is being reduced from 12 weeks to eight at present.
While the European Medicines Agency authorisation specifies a gap of eight weeks, Ireland can avail of an exemption from this.
The Health Service Executive will be looking at the operational implications of the recommendations.
The Irish Pharmacy Union has said pharmacies could begin to administer the Johnson & Johnson vaccine to people under 40 "tomorrow".
Pharmacies have access to 60,000 J&J vaccines to administer but are currently only able to give them to people over 50 years of age. 6,000 vaccines have been administered in pharmacies so far, the HSE confirmed.
Earlier the IPU warned that doses were going to waste because it was difficult to replace no-shows at short notice unless vaccines could be given to younger people.
CEO Darragh O'Loughlin said as soon as the HSE issues a revised protocol it will be implemented "that day".
"Pharmacies have the capacity to vaccinate large numbers of people in their own communities," he added.
"Pharmacists want to play their part to re-open society. We hope that the HSE will empower us to do so."
Meanwhile, the chairperson of the High Level Task Force on Covid-19 Vaccination has said that more than four million doses of Covid-19 vaccines have now been administered.
Professor Brian MacCraith also said 350,000 vaccine doses were administered last week, saying this is the highest weekly total to date.
It comes as the Health Service Executive said that due to operational reasons, it may not always be possible to deliver all second doses of Covid-19 vaccines in the timeframes recommended by NIAC.
In response to a query about reported delays in some people getting their first and second dose of a Pfizer vaccine at The Helix in Dublin, the HSE said that appointments may be automatically scheduled up to five days after the recommended date.
"Second doses given within this window are clinically safe and effective," the HSE added.
It said the HSE strives to deliver all second doses in the timeframes recommended.
HSE Chief Executive Paul Reid said that in four out of the previous seven days, more than 54,000 doses were given.
Read more: Latest coronavirus stories
Elsewhere, the National Public Health Emergency Team met today amid rising concern over the increasing transmission of the Delta variant in Ireland.
Last week we administered our highest level of vaccinations to date at almost 350,000. In 4 of the past 7 days we administered over 54,000. Almost 110,000 of these were 2nd dose Astrazeneca. Another strong week ahead based on supplies. @HSELive— Paul Reid (@paulreiddublin) June 28, 2021
There have been 305 cases of Covid-19 reported by the Department of Health.
There are 49 people with Covid-19 in hospitals around the country, with 16 of these receiving treatment in intensive care units.
The Department of Health said that the daily case numbers "may change due to future data review, validation and update".
In Northern Ireland, there have been an additional 211 confirmed cases of the virus.
No further deaths have been reported in the past 24 hours. Sixteen patients are in hospital, two of them in intensive care.
The latest vaccination figures show 2,010,028 jabs have been administered in Northern Ireland in total.
Delta variant set to become dominant coronavirus strain
The Delta variant is on track to become the dominant strain of Covid-19.
In the UK, it accounts for 99% of cases.
In Ireland, 20% of infections have been cased by the Delta strain, and nearly half of all Delta cases have been in the 19-34 age group.
The symptoms of the Delta variant are slightly different, with GPs warning they can be mistaken for hay fever.
"Very often the symptoms can be what people consider to be minor," explained Dr Deirdre Collins, a GP in Kilcullen, Co Kildare.
Some countries have added 'runny nose' and 'headache' to the list of common symptoms.
"At this time of year those symptoms can easily be put down to hay fever and people who suffer with hay fever might assume that's what they have, that's the concern at the moment as regards Delta," said Dr Collins.
Delta is transmitted more easily than the Alpha variant, which was responsible for the third wave at Christmas.
In the UK, there is evidence that a person infected with the Delta variant is twice as likely to end up in hospital.
While hospitalisations are growing in the UK, the overall numbers are low compared with the last wave, an indication that vaccination is working.
Analysis by Public Health England has found that two doses is better than one.
The Pfizer-BioNTech vaccine is 96% effective against hospitalisation after two doses, with one dose it is 94% effective.
The Oxford-AstraZeneca vaccine is 92% effective against hospitalisation after two doses, with one dose it is 71% effective.
Looking to the UK gives some indication about what is coming down the tracks but according to immunologist Dr Christine Loscher there is one key indicator pertinent to this country.
"The most relevant one for us at the moment is that in the 60-69 year old cohort in the UK there has been a five-fold increase in hospitalisations," she said.
"They are a cohort we are particularly focused on here because only 24% of them have had a second dose, and they need that second dose for maximum protection."
Two in five adults in Ireland are fully vaccinated, while two in three have received one dose of a Covid-19 vaccine.
Anecdotally, GPs are not "seeing an upsurge in patients" coming to them with Covid-19 symptoms, according to Dr Collins.
However, she said that since the HSE cyber attack many test centres have been operating on a walk-in basis, so "people could be self referring".
Meanwhile, a new PCR test expected to be approved for use in the coming days will give a better indication of which cases are positive for the Delta variant.
Additional reporting Fergal Bowers, Sharon Tobin, Louise Byrne