'Many people' have faced wait times of over 90 minutes to get through to the Digital COVID Cert helpline, the Department of Health has said.
The helpline opened yesterday, alongside the return of international travel.
While around two million EU certs have been sent out automatically by phone and email to fully vaccinated people, the helpline is the only way for those who've recently recovered from the virus to get their cert.
The helpline is open for those who've yet to receive their vaccine cert.
Since the helpline opened, callers have expressed their frustration at not being able to get through - with some reporting being on hold for two hours or longer.
The Department of Health has now opened a second helpline number, after acknowledging they're dealing with a 'high volume of calls' around recovery certs.
Officials now say they're prioritising callers who are due to travel in the "very near future".
In a statement, the Department of Health said: "Demand over the first couple of days has meant that many callers were waiting over 90 minutes. A new system has been introduced on the system to trigger an advisory to callers if waiting times are expected to exceed 90 minutes.
"We remain focused on improving the service to callers. Please note that people can request their recovery certificate or vaccination certificate at any stage over the coming weeks if not urgent.
"Queries in relation to a certificate of a COVID-19 test obtained privately can be directed to your test provider."
While around 55 agents are currently taking calls, the Department says that should increase to over 90 by the end of this week.
They handled around 1,160 calls yesterday, with over 1,000 more dealt with as of 2pm today.
The helpline can be reached on 1800 851 504 or 1800 807 008.