Some 18,000 people registered for vaccines through an online portal open to those aged 65 to 69 before lunchtime.
The new portal opened this morning and represents a significant step forward in the vaccine rollout process, health chiefs said.
The HSE will now use the AstraZeneca vaccine only for people aged between 60 and 69 going forward, the head of the executive said on Thursday.
The only exceptions will be people in a younger age group who already received their first dose of AstraZeneca, which mainly includes thousands of healthcare workers who started their jabs, and some people in the Group 4 category who have a high risk of mortality or severe illness.
Healthcare workers have a 16-week interval between jabs, while those in Group 4 have a 12-week interval between shots.
HSE chief clinical officer Dr Colm Henry reassured people getting this vaccine that it is effective.
He said a new pre-print study from the UK shows a single dose of AstraZeneca vaccine reduced the risk of hospitalisation by 94% by 28-34 days after the first dose, and said that the very rare blood clots reported by the European Medicines Agency are not a reason to not take the vaccine.
He said: “I would encourage people to talk to their GP, the GP will say what I am saying now the balance of benefit is heavily in favour of them getting the vaccine, which we know now prevents the most serious.”
HSE chiefs mapped out a revised vaccine rollout, based on the changes from this week, including the pause in the use of the Johnson & Johnson (J&J) vaccine.
Ireland was due to receive more than 600,000 doses of this single-shot vaccine between April and June, meaning plans for the vaccine rollout had to be quickly redrawn. The modelling is based on no J&J doses being available in this quarter.
The AstraZeneca doses will be given out through the mass vaccination centres. The HSE estimates there are about 420,000 people between 60 - 69 who have not yet received a vaccine for other reasons on the priority list.
The online registration portal for those aged 65 to 69 opened this morning, with some 18,000 people registering by lunchtime. About 5% of those registered directly using a phoneline, rather than the website.
HSE CEO Paul Reid described the last few days as a "rollercoaster" for the vaccine rollout, due to the changes in AstraZeneca and Johnson & Johnson use, and the increase in Pfizer doses due. Mr Reid said the vaccine rollout had been changed up to 25 times already due to various issues, and is likely to change again.
He said, overall, it has been a good week, largely down to the 545,000 extra Pfizer doses, due by June, which will balance out the loss of AstraZeneca for certain age groups.
There are no updates yet on what will happen with the Johnson & Johnson rollout in Europe, and Mr Reid cautioned all forecasts are based on supplies arriving in the country on schedule.
Health officials stressed the benefits of vaccinations in Ireland's nursing homes, and provided a promising picture on the health situation elsewhere, too.
Last week was the second week in a row without any new outbreaks in nursing homes, the first time since early July this has happened, while just 4% of all long-term care centres, including disability and mental health units, have outbreaks.
There were 184 patients with Covid-19 in hospitals on Thursday morning, which is the lowest figure since October 9, including 48 in ICU.
Mr Reid said the public’s commitment to following the public health guidelines have greatly contributed to this reduction in pressure on the hospitals.