All healthy Irish people under the age of 60 are looking at June for when they finally get their Covid-19 vaccine this summer.
The 65-70 cohort began registering for their vaccines on Wednesday but it will be another two months before those aged 18-60 will be looking at getting the vaccine.
Speaking on Thursday, HSE Chief Paul Reid said: "We always envisaged June for the below-60 age group. That may change but we need to clarify the Pfizer vaccine supply and any change to the Johnson & Johnson vaccine supply that might come forward."
And health bosses have said they "expect and hope" that the Johnson & Johnson vaccine will become available during quarter two.
Use of the vaccine was paused by its manufacturer this week amid concerns from US regulators over rare cases of blood clotting.
Ireland was due to receive more than 600,000 doses of the single-shot vaccine between April and June, meaning plans for the vaccine rollout had to be quickly redrawn.
While these problems were offset by news of an additional 545,000 doses of the Pfizer jab, health bosses are still hopeful the J&J inoculation will be approved for use by the European Medicines Agency (EMA) next week.
HSE chief executive Paul Reid said: "All we know right now is that it has been withdrawn for now, paused, and we can build based on that.
"What we would expect, and we will certainly hope that the Johnson and Johnson does come back within the quarter, with whatever definitions are defined by the EMA or NIAC (National Immunisation Advisory Committee) or the organisation themselves."
Earlier on Thursday, Tanaiste Leo Varadkar said Ireland is on course to lift more coronavirus restrictions from May 4.
The Tanaiste also said that 80% of the population will have had their first dose of Covid vaccine, or have been offered it, by the end of June.
He said the Government will sit down at the end of April to develop a reopening plan for the months of May, June and July.
The Government is planning for more outdoor activities, a phased reopening of retail and the return of personal services such as hairdressers on a staggered basis next month.
He said: "For the first time this year, the number of people in hospitals is below 200 and the number in ICU below 50.
"Cases are stable or falling, the R number is below one. Kids are back to school, the five kilometre rule is gone, we're building houses again.
"We are on track, we're on track both to ease restrictions as planned from May 4, and to have over 80% of people receiving their first vaccine by the end of June."
