The HSE has apologised to people left waiting for up to five hours for their Covid-19 booster shot on Saturday at the Citywest vaccination centre in Dublin.
The centre was overwhelmed by people who had appointments for booster shots, along with those in the 60 to 69 age group who turned up because it was also designated as a walk-in centre.
Long queues for the Covid-19 test centre also on site led to traffic jams in the area.
In a statement, the Health Service Executive said the centre was "extremely busy" and had experienced "unprecedented demand".
It said a local scheduling error meant many people were called for vaccination who were not eligible for one today.
“The team in Citywest would like to apologise to these people and the people who experienced long delays," the statement said.
"Our operations team at Citywest made every effort to lessen the inconvenience and is reviewing operations with a view to improving the flow and throughput at the site.
“The team will be in direct contact to reschedule any appointments that could not happen today. People who did not get their appointment today will receive another appointment.
"Tomorrow there is a vaccination clinic for 60 to 69-year-olds from 8.15am to 1pm and a clinic for health care workers from 1pm to 7.15pm.”