A British plan, requiring non-Irish EU citizens to hold a visa to cross the Irish border, would require 100,000 troops to police it.
That's according to Donegal TD Thomas Pringle, who says the plan will have to be scrapped as it cannot be implemented.
On Tuesday, British MPs voted down an amendment which sought to remove the visa requirement.
Foreign Affairs Minister Simon Coveney said Ireland's concerns were ignored.
On Twitter, he said the move was against the approach of the Irish and British governments.
"This decision is regrettable & contrary to the approach that UK and Irish Governments have supported for many years to protect free movement on the Island of Ireland for everyone.
"Our concern on this has been communicated clearly but has been ignored."
This decision is regrettable & contrary to the approach that U.K. and Irish Governments have supported for many years to protect free movement on the Island of Ireland for everyone.
Our concern on this has been communicated clearly but has been ignored. https://t.co/7OqH9ebwzX
— Simon Coveney (@simoncoveney) March 22, 2022
Donegal TD Thomas Pringle told Newstalk Breakfast this will not be workable.
"It's going to make a huge impact on people's lives, and indeed even for non-EU citizens who live in the six counties as well who cross the border into Donegal.
"There's a lot of people in that situation as well - that the Northern Ireland health services rely on... who are going to be caught up in this situation too.
"It just doesn't make any sense, and it's completely unworkable as well".
He says the British government will have to put thousands of troops on the border to make this work.
"I don't see how it's ever going to be policeable anyway.
"If it is going to be enforced, basically it will require the British to put 100,000 troops back on the border again to ensure it'll be enforced - because there's no other way it can be.
"Coming to the Dáil, I cross the border twice on a Tuesday and twice on a Thursday - so how can it be enforced?"
Put to him that a lack such controls is a gap in the UK's border security, Deputy Pringle replied: "If they're concerned about it, then just leave Ireland altogether - and that would solve the problem once and for all".
And he believes the approach will have to change.
"I think realistically it will change because it's not even implement it.
"I think it's non-sensical - I think even for a British government to propose it and put it forward, and to allow it to go through to the [House of] Commons just doesn't make any sense at all".