Red tape over getting informed consent from people being given the Covid-19 vaccine is among the reasons why it was not immediately rolled out upon delivery of the first batch to Ireland.
The first 10,000 doses of vaccine arrived in Ireland on Saturday but the first patients won't get them until tomorrow - and then only a small number.
A number of European countries began their vaccination programmes yesterday.
Health Minister Stephen Donnelly last night rejected claims this amounted to a delay here, insisting it is being brought forward by a day.
Cabinet will discuss the Covid-19 situation, including the present restrictions, this afternoon amid a worrying rise in cases in recent days.
Four more people have died and 744 new cases were reported yesterday. That's down from the 1,296 new cases reported on Saturday.
But chief medical officer Dr Tony Holohan last night warned of a "deteriorating disease trajectory" and said he expects higher numbers to be recorded when more people get tested post-Christmas.
Meanwhile, HSE boss Paul Reid, told RTÉ that vaccinations will start tomorrow. A small number will be administered in Beaumont, St James's, Cork and Galway University hospitals.
He said Ireland wants to "do this right" and there is a "complex consent process" in relation to the elderly and vulnerable.
In the North, where the roll-out of the Pfizer BioNTech vaccine began before Christmas, GP surgeries have already started to receive a second vaccine by AstraZeneca. The UK's regulatory body is expected to approve its use in the coming days and GPs are drawing up lists of their patients aged over 80 who will receive it first.
The Herald understands that finalising the informed consent process was one of the reasons the roll-out of the Pfizer vaccine did not start here immediately.
There is concern in health circles over the highly litigious medical legal environment in Ireland in recent years.
Vaccines are rigorously tested and side-effects are usually mild. The EU and other jurisdictions that have approved the Covid-19 jab have insisted it meets safety standards.
Dr Rita Doyle, the president of the Medical Council, had expressed her dissatisfaction with the timing of the roll-out on Twitter on St Stephen's Day, asking: "Why are we waiting four days to start vaccinations?" adding "This is an emergency."
Last night she said she welcomed the change to the roll-out plan, with vaccinations due to start tomorrow instead of Wednesday.
"I am a practising GP. Like doctors around this country, in fact like all of us, I am frustrated with this virus.
"I am frustrated that numbers are rising again at an alarming rate," she said.
But she said this was "being replaced by hope" now the jabs will begin tomorrow.
Social Democrats TD Gary Gannon took to social media to claim: "There's an annoying lack of ambition or self-confidence around our vaccination programme."
He said vaccinations have begun elsewhere in Europe while "we have two boxes carried on a forklift to great fanfare lying in a warehouse for four days".
Mr Donnelly argued such criticism was "unfounded".
He told the Herald: "We took delivery on the 26th. We'll be vaccinating on the 29th so the question I would put back is are people suggesting that we don't follow the expert advice?
"Are they suggesting we don't make sure informed consent is correct? We are clinically led. We have to get this right."
Mr Donnelly said training of staff is taking place and the informed consent process is being put in place.
He added a lot of work has gone into making sure that the informed consent process is "legally robust".
Separately, Tánaiste Leo Varadkar admitted that Ireland is slower in rolling out the vaccine than some other countries but promised "we will catch up".
He said roughly 40,000 doses will be arriving every week from January.
He said Cabinet will be reviewing the decision to allow non-essential retailers to stay open in the wake of a National Public Health Emergency Team recommendation that they should be closed.
Meanwhile, the National Virus Reference Laboratory (NVRL), the largest medical virology laboratory in the country, closed on Christmas Day and will reopen tomorrow.
An 'on-call' service has been in operation since December 25, but Covid-19 result requests cannot be accommodated over the four-day period of closure.
The lab is also closed for three days from January 1 to January 3. The HSE said the NVRL normally processes around 700 swabs per day.
In a statement, the HSE said: "The HSE processes between 10,000 and 20,000 tests per day based on demand. The NVRL were not required from Friday 25th to Tuesday 29th.
"This has no negative impact on testing responses as we are still working within our available capacity."