Checks on goods and animals may need to be carried out close to the Irish border in the event of a no-deal Brexit, the Taoiseach has warned.
Leo Varadkar outlined the impact a no-deal departure would have if the UK cannot strike a new deal with the European Union.
The premier was speaking at an event in Dublin on Thursday night.
Mr Varadkar said checks would take place 'as far as possible' in ports, airports and businesses.
"We don't wish to see a no-deal Brexit and we will continue our efforts to avoid one, but not at any cost.
"Unlike some, I see no upsides to no-deal. I do fear it. But I am prepared for it."
The Taoiseach told a British Irish Chamber of Commerce dinner that his government is "working out the details" with the European Commission in terms of the impact of a no-deal Brexit.
However, he said the common travel area between Britain and Ireland will remain in place, but outlined the changes he expects.
"There will still be plenty of food on shelves but perhaps not all of the same brands," he said.
"When you fly into Ireland from Britain, you will no longer pass through the blue channel.
"You will have to choose the green or red one and pay any taxes that may be due.
"The same will apply to products bought online from the UK, brought in from Northern Ireland, and EU consumer protections will no longer apply."
The Taoiseach added that the recent turmoil at Westminster made him believe there was a 'growing risk' of the UK leaving without a deal.
However, he added that the close relationship will remain between Ireland and the UK no matter the outcome of Brexit.
"The UK is geographically and culturally our closest neighbour and will continue to be a vital trading partner, no matter what shape Brexit takes," he said.
SDLP leader Colum Eastwood said Mr Varadkar's comments "underscore the need for a backstop to prevent the re-emergence of a hard border in Ireland".
"Taoiseach Leo Varadkar has clearly laid out some of the immediate challenge border communities and businesses will face in the aftermath of a no-deal exit.
"It is uncomfortable listening for people who rely on frictionless travel and trade across this island," he said.
"The warnings issued this evening again underscore the critical need for a backstop to protect the interests of people, communities and businesses on this island."