Professor Luke O'Neill has said people in their 60s, who were mainly offered the AstraZeneca vaccine, could be topped up with Pfizer.
It comes as the vaccine rollout may be significantly accelerated because of new research about immunity.
People who have had COVID-19 are now immune for nine months as opposed to six, according to the Health Information and Quality Authority (HIQA).
It means people under the age of 50 who have had the virus recently should only need one dose of a vaccine.
Meanwhile, the gap between doses of the AstraZeneca drug may be reduced from 12 to eight weeks.
If implemented, the proposed changes would see tens of thousands of people fully vaccinated ahead of the previous schedule.
Professor O'Neill told Pat Kenny mixing and matching vaccines is already being done.
"This is one of the key topics in immunology at the moment - should this be allowed - there's no reason not to do it, by the way.
"Other vaccines we often mix and match.
"Spain have decided to mix and match; they've got 1.5 million people there on AstraZeneca [and] they will now be given the option of Pfizer, Moderna as the second shot.
"They can stick with AstraZeneca if they want, but they'll have a choice now... and it's the first country to allow that.
"Canada have just said they're allowing Pfizer as a second shot after AstraZeneca, so suddenly people are waking up to this as a possibility."
Speeding up vaccines
He said this approach could see a better response, and would also speed up the entire vaccine process.
"I think in Ireland especially, the over-60s have been so put upon and many, many would like Pfizer - and I would definitely support that now.
"What Spain and Canada are saying is there's no reason not to mix and match, which is a really, really interesting development."
And he added that this plan is boosted by the large numbers of Pfizer now in Ireland.
"I would give all the over-60s Pfizer as a second shot - we now have 284,000 doses of Pfizer, Brian McGrath announced yesterday, has just come into the country.
"We've a massive supply of Pfizer, and that's what the Canadians did.
"They realised they'd loads of Pfizer - 'let's give it to people as a second shot after AstraZeneca'.
"I don't see why we can't do that here, especially in the over-60s because they do feel very put upon."
Meanwhile, anyone aged 43 can register for a vaccine from today as the rollout continues.
