Certification of medical fitness to drive must be provided by a medical practitioner registered in the State.
The regulations on driver licensing require medical certification of fitness in a number of circumstances. In most cases, this must be done by a person who is registered as a general medical practitioner. In some specific cases, e.g. with endocrine disorders, a specialist opinion is required.
It is a requirement that medical practitioners who certify medical fitness to drive must be registered in the State so that their qualifications can be officially verified.
If a doctor working in Northern Ireland is also on the State register, then they could of course certify fitness to drive.