Tens of thousands of Irish residents who use UK driving licences will not legally be able to get behind the wheel of their car on November 1 if there is no Brexit deal.
Some 40,000 people who live and work in the Republic have yet to exchange their UK licence for an Irish one which will be recognised across the EU.
Until Brexit Day, the switchover to an Irish licence is a relatively simple process that takes 10 days - but if the UK crashes out of the EU, it will become much more complicated.
"Just like other third countries such as South Africa and Russia, you will have to go through a very different process," Road Safety Authority (RSA) chief executive officer Moyagh Murdock said.
Drivers will have to take theory tests, driving lessons and also eye tests again to have an Irish licence, she said.
The RSA is warning that any Irish resident who continues to drive on a UK licence could face prosecution. However, Ms Murdock said the real danger lies in the implications for anybody who is involved in an accident.
"If you are involved in any kind of incident on the road and you cannot produce your documents, well then that kicks in a whole lot of other issues around the actual enforcement of an investigation within that collision," Ms Murdock said.
She noted that an insurance company may not be obliged to honour a policy if the driver is on the road illegally.
The Withdrawal Agreement as it stands provides for arrangements that would recognise UK driving licences across the EU during the Brexit transition period, which is until December 2020.
However, if it is not passed by the House of Commons, then the EU will no longer acknowledge UK licences.
Visitors or tourists who are coming to Ireland from the UK for a short period or holiday will be able to drive using their existing licence.
"While a significant number of people have changed their UK or Northern Ireland driving licences to Irish licences - around 30,000 so far this year - there are still an estimated about 40,000 people driving and living in Ireland who, by October 31 in the event of a no-deal, will be driving illegally," European Minister Helen McEntee said.
"We would encourage people to change over. It's a process that takes no longer than five to 10 days. It's €55."
Efforts to avoid a no-deal scenario will continue today when Taoiseach Leo Varadkar meets British Prime Minister Boris Johnson in New York.
Mr Varadkar will warn that Ireland will not accept "some sort of halfway house" alternative to the backstop to avoid a hard border.
Speaking last night, Mr Varadkar said the UK will be to blame if there is a return to some form of border checks in the event of a crash-out scenario.
He also insisted that Ireland will not cave on the need for the backstop or realistic alternatives. However, he added: "We will work until the very last moment to avoid no deal, but not at any cost."
Responding to British claims that Ireland will come under pressure from the EU to compromise in the final days before October 31, Mr Varadkar said: "One thing I do know about Brexit for the last two or three years, and there are some people in Britain... who took the view that sooner or later, the French, the Germans, the big countries would gang up on Ireland - and that's never happened."