Limited volumes of the coronavirus vaccine will be available in Ireland in January and February, Taoiseach Micheál Martin has said.
These will be used to protect the most vulnerable such as nursing home residents and healthcare workers.
The vaccine will be available in Ireland one week after it is approved at a European level.
The Taoiseach said the "optimal period" for the rollout of the vaccine will come in May and June.
He said: "January and February will have limited volumes of the vaccine. They will go to the priority areas that we've identified.
"We will have enough for the areas of priority. The bigger volumes will come in March and April and May, as other vaccines come on stream and as the manufacturing of vaccines ramps up across the world."
He added: "I think your optimal period is moving towards the May/June period. Then it will be completely open after that."
The Pfizer/BioNTech vaccine is expected to be approved by the European Medicines Agency (EMA) by December 29 at the latest. Approval for the Moderna vaccine is expected by January 12.
However, Mr Martin said the arrival of the vaccine did not mean people should let their guard down.
He said: "The key point is this, the vaccine is a complimentary tool to the measures we are already using in terms of restrictions.
"People need to be aware of that, just because a vaccine is arriving, doesn't mean we can let down our guard.
"It will add significantly to our armoury in terms of putting the pressure on this virus."
The Taoiseach has admitted he is concerned by the recent Covid-19 figures in Ireland, with 429 cases reported on Sunday.
He said it was possible further restrictions will be introduced in January after the Christmas period.
He told RTÉ's Morning Ireland: "Of course we know how this can grow exponentially, the virus can grow. I am concerned about it.
"We had six weeks of severe restrictions. I think it was only natural and the right thing to do to ease up on those restrictions to some degree.
"The reality is, as I said at he end of those six weeks, personal behaviour is essential now as we move into the Christmas period. We've all got to watch our contacts. Every social contact counts here.
"You could very well be looking at some further restrictions in January."
The Taoiseach said the coronavirus figures will be kept under review.
Those wishing to meet their family at Christmas, he said, should begin cutting their social contacts now.
He said: "The most effective response to this is our own personal behaviour.
"If people want to see their loved ones, their parents and their grandparents, it's very important that next week we reduce the number of social contacts that we have."