The Taoiseach has hailed "a day of great hope" as the first coronavirus vaccine arrived in Ireland on St Stephen's Day.
The Pfizer-BioNTech vaccine is the first to be approved for use in Ireland by the European Medicines Agency.
The initial batch of 10,000 doses will begin to be administered on Wednesday.
The first delivery of #COVID19 vaccines have arrived in Ireland - a day of great hope as we head into 2021.— Micheál Martin (@MichealMartinTD) December 26, 2020
Those who work in our health services deserve huge credit as they swiftly and safely begin the roll out to the most vulnerable in our society. pic.twitter.com/lTiVkWbo1m
Micheál Martin wrote on Twitter: "The first delivery of #COVID19 vaccines have arrived in Ireland - a day of great hope as we head into 2021.
"Those who work in our health services deserve huge credit as they swiftly and safely begin the roll out to the most vulnerable in our society."
Health Minister Stephen Donnelly said that vaccinations will begin in four days.
The Minister posted a picture on Twitter of himself standing next to the ultra-low temperature fridges needed to store the vaccine.
He wrote: "When is a fridge worth photographing? When it's just had Ireland's first #Covid vaccines put in it !The first doses have just arrived and many of them are sitting in that very, very cold fridge. We'll begin vaccinating in four days."
HSE chief executive Paul Reid described Saturday as a "momentous day" for the country.
He tweeted: "An early morning start to a momentous day. Heading off to take receipt of the first delivery of the Pfizer BioNTech vaccine for the HSE. There will be better days ahead for sure. For now, #StaySafe"
Mr Reid later posted a video of the vaccines being transported in a warehouse.
He said: "Its arrived! Taking delivery of the Pfizer BioNTech vaccine into our Cold Chain Storage this morning.
The vaccine is to be supplied to the most vulnerable in society first, such as nursing home residents.
However, Taoiseach Micheal Martin has warned it will be up to six months before things start to return to normal.
He said: "The first six months of 2021 we will see improvements, but we certainly not will see normality in the first six months."
Mr Martin said the initial phase will make a "significant difference" and protect the most vulnerable.
He added: "Certainly manufacturing of the vaccine will be ramped up, certainly from March onwards."
Mr Martin said May and June have been identified as "critical" months, adding: "From the summer on we will see a degree of normality but I cannot be definite about that."
He said recent concern over the emergence of a mutant strain of Covid-19 in parts of England highlight the continued uncertainty.
The Taoiseach expressed confidence in vaccine supplies in the long-term.
"By the end of January you will have three vaccines, and what I am saying is conservative.
"I can also see a scenario where manufacturing ramps up more quickly and where higher volumes of vaccines goes to member states more quickly, that is a more hopeful scenario."
The State entered its third lockdown on Christmas Eve as a range of restrictions were introduced.
Bars, restaurants, hairdressers and beauty salons have shut in a bid to stifle the spread of coronavirus.