Social Democrats co-leader Catherine Murphy has called for an assurance from the Government that the Health Service Executive's booking system for Covid-19 vaccinations is working properly.
It comes after Taoiseach Micheál Martin said that only 93,000 people turned up for boosters last week when 180,000 people had been invited.
Deputy Murphy said: "Eligible people can now get boosters from GPs, the pharmacies or indeed the vaccination centres - the only problem is that the systems don't seem to be capable of talking to each other."
She told the Dáil: "Instead of blaming people for not turning up to their appointments, can you tell us what's being done to resolve the problems with the booking system."
In reply, the Taoiseach said that he was not trying to blame people, but underline the importance of the booster campaign - particularly given the emergence of the Omicron variant.
He said: "There really has to be a very clear message from all of us - when you get your opportunity to get your booster, please take it."
Mr Martin said the HSE was "working through their systems ... and they're constantly reviewing how best to maximise uptake in the next few weeks".
Earlier, the HSE said people may be getting several appointments for a Covid-19 booster vaccine from multiple sources, even if they have already been vaccinated.
It said that while steps have been taken to minimise this, it is inevitable this will occur but its priority is to ensure the HSE gives people maximum choices to get vaccinated.
The HSE said that people can inform it online, or by text, that they have received their Covid-19 booster elsewhere, or reject a booster appointment if they have already received a third dose.
It said that in order to give people maximum opportunity to get a booster and to use all capacity in the programme, the HSE is using vaccination centres, GPs and pharmacies as vaccination channels.
It said that in comparison to the primary vaccination programme, it is taking longer to vaccinate people in some groups with the boosters.
It added that this is evident in lower attendance rates at both scheduled clinics, and at walk-in clinics.
"Even allowing for acknowledged issues with notification across the different vaccination systems, it is evident that it is taking longer for people to attend for the booster vaccination.," the HSE said.
It has advised that if someone has already received a booster Covid-19 vaccine dose and wants to cancel an appointment from the HSE, or if they plan to get a booster dose through a GP, or pharmacy, they should reply to the text received from the HSE with the word 'REJECT'.
They can also use an online form on the HSE vaccination registration portal to cancel an appointment scheduled by the HSE.
Meanwhile, the HSE National Director for Test, Trace and Vaccination has said that there have been some challenges with multiple appointments being issued for Covid-19 booster vaccinations due to a time lag between records being registered on the central system.
Damien McCallion said unlike the primary vaccination programme, the HSE took the decision to open all channels at the same time as vaccine centres, GPs and pharmacies and he says it has "led to some issues in relation to the systems".
"We do recognise that for some people it has been a little bit frustrating where they have multiple appointments and they may have already been vaccinated through another source, through a GP or pharmacy," he said.
Mr McCallion said there are number of options to enable people who have already received their booster to cancel additional appointments that are issued.
He said the HSE's IT teams are working to improve the systems and minimise multiple bookings.
A system has been set up, he said, so that when a person is administered their booster jab in a GP surgery or pharmacy, the record "will automatically cancel or avoid scheduling appointments" for that person.
HSE Chief Operations Officer Anne O'Connor said that over one million extra doses of vaccine have now been administered.
Booster vaccines will open from tomorrow to those aged between 50 and 59.
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Tánaiste Leo Varadkar said the Government is trying to get more information about the low take-up of booster vaccines.
"I wonder if there's problems with the booking system. There's definitely some issues we need to iron out," he said.
Separately, a General Practitioner said she does not believe that people are failing to turn up at vaccination centres for booster shots, but that they are getting their shots in a different place instead.
Speaking on RTÉ's Today with Claire Byrne programme, Dr Maire Finn said that there are three options for patients to receive a booster: vaccination centres, GPs' clinics and pharmacies.
She said she thinks the issue is that people are getting boosters elsewhere but are unable to cancel their HSE appointment.
"Maybe the systems don't really speak to each other as well as they should," Dr Finn said.
Nursing home receives appointments for deceased residents
The Director of Nursing a nursing home in Cork has said they have been receiving texts from the HSE offering booster vaccines for residents who are deceased.
Janet Woodward, from the Douglas Nursing and Retirement Home, said she has spent hours on the phone, speaking to different people who say they have now taken the deceased people off the list but the home is still getting texts with appointments.
As Director of Nursing, she receives texts for booster appointments.
She said that when she sought to use the online HSE form to indicate an appointment is not needed, and to cancel a booster appointment, the system asks for a date and a place that the resident got the booster.
Ms Woodward estimated that this has caused at least 24 wasted appointments for just that nursing home.
Additional reporting Fergal Bowers, Fergal O'Brien