A significant under-delivery of the Johnson & Johnson vaccine has put the Government's vaccination timeline in doubt.
Stephen Donnelly said the "best-case scenario" is that Ireland would receive just half of the expected one-dose vaccines, but the actual delivery could be much lower than that.
There are also "question marks" around AstraZeneca vaccine amounts.
It's currently 'unclear' whether 80% of adults will be offered a vaccine by the end of next month.
Registration for 40-44 year-olds is expected to open soon - with an exact date still being finalised, particularly in light of the Johnson & Johnson supply issues.
However, Minister Donnelly said the country is now on track to have vaccinated half of all adults by the end of this week.
He said the vaccination programme has continued uninterrupted despite the cyberattack targeting the HSE and Department of Health.
However, validating the exact data has not been possible.
The minister said: “It is estimated we have administered in excess of 2.5 million doses of vaccine.
"The exact number right now is not possible to provide, as the data from GPs across the country cannot be reported back.”
Nonetheless, he said around 280,000 doses are being administered this week - although that number may be even higher.
Minister Donnelly said the Government is moving “ever closer” to the goal of offering vaccinations to everyone who wants one, with Pfizer supplies said to be particularly reliable.
He suggested it's still ‘unclear’ whether there’ll be enough vaccine supply to meet the Government's target of offering vaccines to 80% of adults by end of June - noting the serious concerns around Janssen vaccine supplies.
The issues are said to be around a manufacturing site in the US, with the EU waiting for necessary approval from US regulators.
Ireland was expecting around 475,000 doses of the Janssen vaccine in June - but the 'best case' is around half of that.
However, the worst-case scenario could see just 60,000 doses delivered.