Many callers have complained that they cannot get through to the new helpline number for queries on the Digital Covid Certificate.
The Department of the Health said it is working to increase capacity on the helpline but blames a "very high number of calls" for the difficulty on its first day of operation.
The number 1800 851 504 was provided for queries and for those who had recovered from Covid to apply for the cert.
Those who have been fully vaccinated are supposed to receive their certs automatically.
However, members of the public have complained that the helpline number appears to be down.
A spokesperson for the Department of Health acknowledged that people may have difficulty getting through.
They said: "We are currently working to increase the capacity of the service centre.
"The helpline number is 1800 851 504 and is for queries relating to your vaccination certificate and/or requesting your recovery certificate."