Some second dose vaccine appointments are now being offered just 17 days after the first, the HSE has confirmed.
They've also said anyone who may have received an appointment for less than 17 days after their first dose should reschedule their appointment.
Until recently, it was recommended that Pfizer vaccine doses be offered 28 days apart.
That advice has now been changed to 21-28 days for the Pfizer jab.
However, some people have been receiving second appointments even earlier than 21 days.
Under official guidelines, any dose given less than 17 days after the first is considered 'invalid' and a third dose should be administered 28 days later.
According to the HSE, however, a reduced gap of 17-20 days is viewed as "clinically safe and effective".
In a statement, the HSE said: "We strive to deliver all second doses of COVID-19 vaccines between 21-28 days after the first dose, however we are in a position to offer the second dose after 17 days in some cases.
"A number of operational and logistical factors could enable this, including supply of the vaccine, the number of people that have registered and are being assigned appointments to a particular vaccination centre. Second doses within this window are clinically safe and effective."
Anyone who receives an appointment scheduled for less than 17 days after the first can reschedule by following the link in their appointment confirmation text message or call the HSE's helpline on 1800-700-700.
Other vaccines have longer recommended gaps than the Pfizer one - including four to 12 weeks for AstraZeneca, and 28 days for Moderna.
It comes as walk-in vaccination centres open across the country this weekend for people aged 16 or older who've yet to receive their first dose.