The Northern Ireland COVID Certification Service system was offline for several days due to technical issues but resumed on Saturday evening.
Those hoping to travel between Wednesday, August 4 and Tuesday, August 10 inclusive are now invited to apply.
Some applications will be processed manually and applicants will be contacted via email on what to do as they progress.
The Department of Health says that while the service has resumed, it is being managed in a controlled and phased way and will be strictly limited to the dates highlighted.
The first process will involve proof of identity. with 87 per cent now progressing through this step automatically.
The remainder are validated manually. Those requiring manual verification are being processed as a priority.
Once applicants have completed the proof of identity checks they will notify you directly by email, telling them when you can go online or on the app to request a certificate.
“We would appeal to those travelling later than August 10 that they do not apply at this time and help us help their fellow citizens,” said a spokesperson for the Department of Health.
Further communication will follow in relation to those travelling after August 10 with an update on plans to extend the periods when applications can be accepted.
The Department has apologised for “the inconvenience caused at what is undoubtedly an already stressful time for many of our citizens”.
Information on the Covid Certification Service is available on NIDirect website.