People aged 65 and over and immunocompromised people aged 12 and over are being encouraged to avail of a second Covid-19 booster vaccine.
The rollout of the second dose was recommended by the National Immunisation Advisory Committee (NIAC) earlier this month.
It is estimated that there are around 800,000 people in the two cohorts identified by NIAC as among the groups at highest risk from the virus, who are currently eligible to receive the fourth dose.
Appointments for vaccination centres can now be made online through the HSE website, www.hse.ie, while participating pharmacies will begin offering the second dose from early May.
It will also be available in the coming weeks through GPs, who will make contact with their patients.
The National Director of HSE Vaccination Programme, Damien McCallion, said multiple channels are being offered to maximise the take-up of the second booster and people do not have to wait to be called by the doctor.
He said "people will be contacted by their GP, they don't need to contact their GP, but yes they can book an appointment at a vaccination centre in the meantime".
"On the system, we try to knit that information together to ensure that we have minimised the disruption for people and for general practitioners, pharmacists and our own vaccination centres."
For anyone getting the second booster jab, there must be a gap of at least four months since their last booster, or in cases where they recently contracted the virus, they must wait for four months from the time they tested positive.
NIAC is continuing to examine the evidence on whether other groups should also be offered a second booster.
Mr McCallion said at this stage there are no defined plans, but he said "clearly they will always look at the evidence, look at what is happening in other countries, look at the research and base their recommendations on that".
"So, we will link closely and we will inform people if that situation arises and we will always do contingency planning in the HSE in the event that advice comes, so that we can turn the advice quickly into an operational plan."
A leading immunologist said he believes a second Covid-19 booster for people aged 65 and over and immunocompromised people aged 12 and over is "certainly a good idea".
Professor of Experimental Immunology at Trinity College Dublin Kingston Mills said: "Immunity wanes after the first booster, so getting a second booster would be advantageous in terms of preventing severe disease and hospitalisation."
He said while the number of new virus cases and the number of people with Covid-19 in hospital are falling, the additional dose would be beneficial to those in higher-risk categories.
Prof Mills said: "The figures are certainly going in the right direction but there are still a number of people, a couple of thousand a day getting infected, and some of those are still ending up in hospital."
As a result, he said: "It's important, especially for the vulnerable groups, the over 65s and those with underlying medical conditions to get a second booster now to put them in a better position to withstand the effects of the latest variants."
However, he added he does not believe there is a need yet to roll-out a fourth dose of the vaccine to the general population.
Prof Mills said: "The disease is relatively mild in the younger population and with Omicron the disease is milder than it was with Delta, so the risks are not as great for the younger population and healthy people as it was when we had Delta and the previous variants."
Almost 2,800 Covid cases reported today
The Department of Health was notified of 2,778 new cases of Covid-19. The figure includes 1,171 PCR-confirmed cases and 1,607 positive antigen tests which were registered through the HSE portal.
The number of Covid-19 cases in hospital has fallen below 600 to 599. It is a reduction of 55 cases on yesterday's figure of 654.
There are 39 people in ICUs across the country with the virus.
A new analysis of Covid-19 vaccination programme by the Central Statistics Office has found that in 90% of Local Electoral Areas (LEAs) the vaccine uptake rate among over 12s is greater than 83.9% of the population.
In contrast, the rate of five to 11-year-olds is less than 23.4% of the population.
The CSO data also showed that the booster vaccine uptake rate in LEAs ranged from 52% to 75%.
