Anyone aged between 65 and 69 can now book their Covid-19 vaccination appointments at one of NI's seven regional vaccination centres.
Until now only health care workers have been vaccinated at these locations.
Shortly after the booking site became active on Wednesday night, some users posted on social media that they were experiencing issues getting appointments booked.
In a tweet, the Department of Health's Chief Digital Information Officer, Dan West, said this was "due to high levels of traffic".
He added: "Delays in SMS/email causing time-out of verification. We've temporarily extended validity of codes and added extra capacity. Bear with us while we get this sorted."
Some users experiencing issues tonight with the verification on the booking website due to high levels of traffic. Delays in SMS/email causing time-out of verification. We’ve temporarily extended validity of codes and added extra capacity. Bear with us while we get this sorted.— Dan West (@WestDan_) January 27, 2021
The Department of Health this week announced a twin track approach to accelerating NI's vaccination programme.
It said this would ensure more people can take up the offer of vaccination.
GPs will be focusing on the 70 plus age group, while the seven regional vaccination centres will in parallel be offering offer vaccination appointments to 65-69 year-olds.
As of Monday, 159,642 people in Northern Ireland had received a first coronavirus vaccine dose.
The development comes as Northern Ireland's Department of Health has recorded 16 more coronavirus-related deaths, taking its toll so far to 1,779.
A further 527 people have tested positive for the virus in the past 24 hours.
Fall in ICU patients
There are 775 people in Northern Ireland's hospitals who are being treated for the virus - 68 of them are in intensive care and the number of people requiring ventilators has risen to 56.
Since Monday, there has been a reduction in the number of inpatients, patients receiving intensive care treatment, and the number of patients ventilated.
In the Republic of Ireland, 54 more Covid-19 related deaths were recorded on Wednesday. It brings the Republic of Ireland's death toll to 3,120.
The Irish Department of Health also confirmed 1,335 more Covid-19 cases.