The head of the National Immunisation Advisory Committee said it is hoped that the "bulk" of the population will be vaccinated against COVID-19 by the summer.
All nursing home residents and staff are to be given the Pfizer/BioNTech vaccine in the next two weeks.
Meanwhile, the first doses of the Moderna vaccine will arrive in the country in the coming days after the jab was given the green light by EU regulators last week.
Professor Karina Butler, member of NPHET and the high-level COVID-19 vaccination task force, said today that a number of measures have facilitated the accelerated roll-out of the programme.
She told On The Record with Gavan Reilly that it has been going "really well" in hospitals this week.
The situation with the virus is "changing all the time", which makes it difficult to give an accurate assessment of when the entire population can expect to have received the jab, Professor Butler said.
However, she estimated: "Hopefully, by the summer we will have the bulk of the population immunised but depending on supply, that might stretch out later than that.
"I personally can't tell you when each cohort might get vaccinated.
She continued: "I know everything is happening here to speed up the roll-out, and there have been so many ways already in which steps have been taken on that.
"When we started at the very beginning, we thought we had to hold back 50% of the supply to give everyone their second dose of vaccine in 15 days.
"Then as an extra supply of vaccine came in, then we could afford to bring that down to a week's buffer.
"Now as the vaccine supplies have been secured, we've been able to cut back on that even further and that has facilitated roll-out."
Professor Butler added: "When we were starting off we thought we could get five doses of vaccine from each vial.
"Now it's pretty commonplace, not every time but pretty commonplace, to be able to get six does of vaccine from every vial.
"What we're concentrating on is how many complete doses, it's a 0.3ml dose, you can get from a vial that has been properly diluted."
She said that in some instances, up to seven doses can be extracted from each vial.
Professor Butler stated that next week, the number of nursing homes involved in receiving the vaccines will increase "very significantly".
"We know that it was hoped that all healthcare workers and all residents of long-stay institutions would be vaccinated, the original target was the end of February," she said.
"That now is being shortened and we also know it's already begun to reach out into some areas into getting some fo the community GPs into those hubs."