Proposals for EU-wide Digital Green Certificate for travel could become problematic for a number of reasons, according to the Irish Council for Civil Liberties.
Further details are expected from the European Union soon after it announced the 'three boxes' approach last week.
The document can be used as proof that a person has been vaccinated against COVID-19, received a negative test result or recovered from the virus.
The pass will be available in digital or paper format for free to all EU citizens and is expected to be ready before the summer.
The Union added that the certificate will not discriminate against people who have not yet received the vaccine.
However, the Executive Director of the Irish Council for Civil Liberties Liam Herrick believes that this could be made more difficult because of a slow vaccine rollout or transmission worries.
Speaking to Newstalk Breakfast with Susan Keogh, he said: "There are two aspects to the issue of vaccine or immunity certificates, one is the question of travel and that's a complex one because we're waiting at this stage for the EU detail.
"Certainly it's possible for states to require certificates of immunity in some circumstances but really the devil will be in the detail."
The ICCL wrote to the Minister for Foreign Affairs last week, calling on him to pay "close attention" to the issues of privacy and discrimination and how they can be avoided with the document.
"A separate question is that some countries, for example, Israel, have introduced immunity or vaccine certificates within the state in terms of access to goods are services," Mr Herrick said.
"We think that is wholly inappropriate and it would lead to a two-tier society, especially when there's a shortage of vaccine so we think this is an opportunity for the Government to come out and say it opposes that. "
He stated there has been disappointment about the vaccine delivery in Ireland and that "in a context of any shortage of supply, a system of this type becomes particularly problematic".
This is because there may be a significant proportion of society who want to be inoculated but can't.
"You're talking about significant proportions of the population who might wish to get a vaccine not being able to access it," Mr Herrick added.
"The WHO has raised some concerns about travel systems of this type, even without that shortage question.
"First of all, there's a question mark over people who have had the vaccine, while they won't develop the disease, the question of transmission on is not 100% accurate and that's a challenge in and of itself."
He also raised the issue that if the digital certificate becomes essential for travel, people may want to get the document as soon as they have been immunised, meaning the vaccine delivery programme could be slowed down.
"A crucial issue here will be people who haven't got the vaccine, can they travel with proof of a negative PCR test or will they also have to get this digital certificate," he said.
Mr Herrick said he also wrote to the Government to ascertain their position on the certificates and establish their red lines.
"I think one of the big concerns that's coming up across Europe in regard to this proposal is that COVID is a short-term issue, hopefully this will be resolved over the next few months or by the end of the year," he said.
"But if you're putting in place a digital certificate system, not talking about people turning up with a paper test proof but a digital system of health surveillance and controls around travel in the EU that once it's put in place, it may not be that easy to unpick.
"There will be a lot invested in putting it in place, and are we looking at a long term health surveillance system across Europe, and I think we need to be very careful to guard against that."
Mr Herrick added that "short term measures can have long term consequences".
Speaking on the same programme, Paul Hackett, the CEO of Click & Go, acknowledged that there are "personal interest" issues to be considered but it appears the EU has taken a "well-considered, coordinated approach to resume international travel safely at some point in the future".
"If there is resistance to taking the vaccine, you will be required to prove that you are COVID negative or you will be required to prove you had COVID but are now recovered," he said.
"There is an element of intrusion into privacy but the proposals also set out they should include a minimum set of information necessary to confirm these particular areas.
"I think the EU are being considerate of countries where there is vaccine hesitancy, they're not calling this a passport, it's a certificate, they are looking to get the minimum amount of information, this will be managed through a QR code system or in paper."
Mr Hackett that if the country is going to have 80% of the population vaccinated by June, as the Government plans, and if we have a proposal for digital green certificates, "it would make sense that we reopen travel".
"There are 400,000 jobs in terms of the aviation sector and the hospitality and travel sector that are impacted by this," he added.