Most Irish passport holders who live in Northern Ireland will not be able to use the EU Digital Covid Certificate.
That is in spite of Taoiseach (Irish Prime Minister) Mícheál Martin and other senior Irish government ministers indicating that they would.
The vaccine "passport" to facilitate international travel comes into effect in the Republic of Ireland on Monday.
It will make journeys within the EU easier for people who are fully vaccinated against Covid-19.
Last week, more than two million people in the Republic of Ireland got their certificates, which contain QR codes, by email and post.
But most Irish passport holders in Northern Ireland will not be able to access the certificate because they were vaccinated outside of the Republic of Ireland.
That means the Irish state has no official record of their vaccination, according to senior Irish government sources.
Making the certificate available to Irish passport holders in Northern Ireland was discussed by the taoiseach and other ministers last week.
Irish Foreign Minister Simon Coveney has said the matter would be "kept under consideration".
In Northern Ireland, the Department of Health has said it is confident that an "international travel app" will be ready "early [this] week" for those who are fully vaccinated.
The EU's vaccine "passport", sometimes known as the Digital Green Cert, has also been made operational in Switzerland, Norway, Iceland and Lichtenstein, which are non-EU countries.
EU citizens and those of Switzerland, Iceland, Norway and Liechtenstein can download it or obtain a paper copy at no cost.
It is also available to non-EU nationals living legally in member states who have the right to travel to other member states.
The certificate can be issued if someone has been vaccinated against Covid-19, recently had a negative PCR test or recently recovered from Covid-19.
The certificate will be accepted as proof of full vaccination in order to gain access to indoor hospitality in the Republic of Ireland.
Irish Tourism Minister Catherine Martin indicated that official UK proof of vaccination would also be acceptable for indoor hospitality purposes, preferably with other proof of identity, such as a driving licence.
However, what is acceptable for indoor dining is not for international travel, according to sources in Dublin.
Those travelling abroad even, with the certificate, are being asked by the authorities to be aware of the entry requirements of the countries to which they are travelling.