Micheál Martin has accused Britons of "backward-looking nationalism" over their decision to pull out of the European Union.
The Fianna Fáil leader has called for an urgent national plan to deal with the "slow-motion crash".
Mr Martin has also demanded the EU suspend normal State aid rules for worst-hit Irish industries.
"We are not going to join the English in their desire to repeal the 20th century," he said.
"We will not join them in their right-wing ideology of trade rules with no social dimension and no enforceable laws."
Mr Martin said Ireland desperately needs a new urgency and ambition in Government to deal with the fall-out of Brexit and other rising threats internationally.
Speaking at a commemoration of Sean Moylan, an IRA commandant in the 1920s and later government minister, Mr Martin said: "Britain has taken the route of a backward-looking nationalism, suspicious of outsiders and committed to the historically false idea that you don't need strong international bodies to secure lasting cooperation and prosperity between nations."
He added: "In the five months since the UK's Brexit vote the only things which are clear are that their policy is a shambles and that it is already causing real damage on this island.
"Brexit is not something which is happening in two years, it is happening now."
Mr Martin said Ireland urgently needs a strategy to mitigate the impact of "the hard Brexit which is already under way".
"The unprecedented decline in Sterling may soon be followed by new barriers to trade," he said.
"We can't stand by and let this slow-motion crash happen."
