The head of the HSE says it'll be "extremely challenging" to meet the target of vaccinating all over 70s against Covid by the end of March.
The new guidance also means some people lower down the priority list will get their jab sooner than planned.
The advice to give over 70s the Pfizer or Moderna vaccine - rather than the AstraZeneca one - has big implications for the rollout.
Those vaccines are harder to store and the HSE says it's unlikely individual GP surgeries will be used to vaccinate that cohort.
Back-up plans being worked on will involve multiple GPs administering doses in the one place.
HSE CEO Paul Reid says getting to all over 70s by the end of March will be difficult.
The changes mean that once the AstraZeneca vaccine arrives next week, people further down the priority list could be bumped up.
The latest update shows just under 58 thousand Covid jabs have been administered in the last week - around 8 thousand per day.
It brings to more than 219 thousand the number of people who've received at least one dose.
The HSE says daily vaccination figures will start to be published from next week.
