Every day an estimated 30,000 cars cross the border as people go to work, school or hospital appointments.
But the big question is whether they will continue to be insured in the event of a hard Brexit.
Despite there being efforts to ensure there is no change after Britain leaves the European Union, an abrupt exit could cause chaos.
"Sheer madness" and "an appalling mess" were just two of the terms used to describe the confusion around post-Brexit motor insurance quandary.
It fuelled testy exchanges in the Dáil chamber. The Minister for Transport, Shane Ross, ended up having to apologise for any offence caused after he called Sinn Féin's transport spokesperson Imelda Munster a "donkey".
Political wrangling aside, the reality is that thousands of motorists could be affected.
What is a Green Card?
A Green Card is an internationally recognised insurance document. According to the Motor Insurers' Bureau of Ireland (MIBI), the body charged with distributing them, it provides proof of the minimum compulsory motor insurance cover required by the country visited. It is a hard copy document which is printed on green paper or with a green background.
Tánaiste Simon Coveney acknowledged this week that green cards have been "slightly unfortunately named".
Will Green Cards be required after Brexit?
If the UK crashes out of the EU or there is a 'no deal' Brexit, then Irish motorists driving Irish-registered vehicles to the UK, including Northern Ireland, will require a 'Green Card' to show the police that they have valid motor insurance. The MIBI said Green Cards will not be needed if the EU and the UK reach an agreement on Brexit or if a transitional arrangement is implemented.
How has Brexit made 'Green Cards' such an issue?
Up to now, Ireland, like all EU members states, was covered by the EU Motor Insurance Directive. This allows motorists from all 28 member states to travel freely between EU countries without requiring any additional insurance documentation. Their domestic insurance is accepted as proof that they are insured to drive in other member states.
If there is a 'no deal Brexit' then the UK and Northern Ireland will be no longer part of the EU's Motor Insurance Directive. Drivers of Irish registered vehicles driving across the Border into Northern Ireland will need to have a 'green card' as proof that they are insured. The same applies for Northern drivers coming south of the border.
Trucks waiting to cross the Turkish Greek border
How do motorists get a 'green card'?
The Motor Insurers Bureau of Ireland (MIBI) has said that it has issued one million Green Card application forms to insurance companies and insurance brokers to issue to policy holders.
When should motorist apply for 'green cards'?
The MIBI have advised any motorist planning on driving in the UK, including Northern Ireland, immediately after March 29 to request a green card from their insurance companies or insurance brokers now.
David Fitzgerald, Chief Executive of the MIBI, said: "Anyone who requests a Green Card will be issued one."
Why has the issue caused such confusion?
Fine Gael TD for Louth Fergus O'Dowd is the Chairperson of the Oireachtas Transport Committee. During this week's Brexit debate he pointed to a possible cause of the confusion around the cards.
He said: "It seems that each different insurance company is following its own rules, there is no united front on it and there is no clarity on it."
He called on Minister Ross to urgently clarify the matter and suggested that all insurance companies send out green cards to all policy holders to avoid the "worst case scenario" where cars are impounded after they cross the Border after Brexit without the insurance document.
The MIBI acknowledged that different insurance companies and brokers are issued the cards in different ways. Some are issuing them to all of their policy holders while others are issuing them to policy holders in select areas e.g. border counties.
Sinn Féin TD Pearse Doherty said that "hundreds of thousands of policyholders are unaware of what is required in terms of a green card when we are just weeks away from a potential no-deal Brexit. "
He, and others including Mr O'Dowd, say that the matter should have been dealt with by way of bilateral agreement between the European Commission and the British authorities so Northern Ireland would be exempt from the green card requirement.
What is Taoiseach Leo Varadkar's position on the issue?
The Taoiseach said that the Government is aware that this is an issue of "great concern" to thousands of motorists who cross the border every day. He said the Government is trying to resolve the matter before March 29.
Mr Varadkar said that the vast majority of the insurance policies which people hold cover the UK, including Northern Ireland. But they will need proof of that, which is in the form of a green card.
What is the 'grace period' referred to by the Taoiseach?
Responding to Pearse Doherty during Leaders' Questions on Wednesday, Taoiseach Leo Varadkar indicated that a grace period will be put in place for motorists after March 29.
He explained that south of the border in the Republic of Ireland, while motorists are required to have a 'green card' as proof of insurance, it is not an offence not to have one.
He added: "There will be a grace period for people who are based in Northern Ireland and coming south and have insurance but not a green card."
"We cannot make a commitment on behalf of the Government in Northern Ireland, because there is not one, and we cannot make commitments on behalf of the UK Government. For people entering this jurisdiction, as long as their insurance policies cover them for this State, even if they do not have a green card on 30 March or April 1 or 2, they will not be prosecuted for it. There will be a grace period for that reason."
The MIBI has pointed out that the grace period referred to by the Taoiseach does not apply to Republic of Ireland motorists who plan on driving in Northern Ireland or the UK, meaning that a Green Card will be required.
Where does the Department of Transport and Minister Shane Ross stand?
The Department of Transport, Tourism and Sport, issued a statement of clarification on the need for green cards on Wednesday evening. It also encouraged people who will need to drive in Northern Ireland or other parts of the UK in the days immediately after Brexit to immediately request a green card from their insurers and brokers.
Mr Ross added: "This is not something that the Government wants to see, rather it is another consequence of a decision by the UK people. As part of its contingency planning, the Government has raised this matter directly with the European Commission seeking agreement from it to set a date from which green cards would not be required as is possible under the EU's Motor Insurance Directive. The Commission has not given agreement to date and the Government continues to pursue the matter with it."
He said his Department is also continuing to request that the EU Commission set a date where green cards would not be required.
This echoes sentiments expressed by the Tánaiste Simon Coveney who said that Ireland cannot resolve this quandary on its own. He said the EU Commission must resolve it on a bilateral and reciprocal basis with the UK. Until that happens, motorists need to be aware that green cards will be required if there is a no deal Brexit.
Where can I get further information about 'green cards'?
The MIBI has further information on the provision of Green Cards to Irish registered motor vehicles in the event that a 'no deal' Brexit taking place here.
