A Senior government official has warned transport chiefs that a crash-out Brexit would present "unprecedented challenges" and told them to be available "at all times" if it happens.
The stark warning came in letters from Department of Transport secretary general Graham Doyle to the bosses of the Dublin Airport Authority, the port companies, Dublin Bus and Iarnrod Eireann among others.
The UK was supposed to leave the EU on March 31, but this was extended after former prime minister Theresa May failed to get her Withdrawal Agreement through the House of Commons.
October 31 is the current deadline for Brexit.
The first extension granted to the UK was up to April 12. It was only a deal struck in Brussels the day before that saw the EU agree to move the deadline to Halloween.
Mr Doyle wrote to state bodies under his department's remit on April 3.
He said Brexit will affect transport, tourism and sport.
"Key risks", he said, had been identified over the previous year, and the department had been working with the various agencies to mitigate them.
However, he warned that "notwithstanding our extensive preparations", if the UK had left the EU on April 12 without some form of formal agreement with the EU, they would be presented with "unprecedented challenges across our sectors".
He said it was "vital" that the department and its agencies are "ready to respond immediately to any issues".
Mr Doyle asked that all state bodies "ensure appropriate monitoring, oversight and decision-making arrangements are in place" and that senior management "be available to be contacted at all times".
The Department of Transport said it was "continuing with its contingency planning to meet the challenges for a possible October 31 no-deal Brexit".