Taoiseach Micheál Martin said it is time for the UK and EU to "turn the corner" on the damage done to relations following Brexit.
At the British-Irish Council press conference in Cardiff, he was asked why UK/EU relations were so poor five years on from Brexit.
Mr Martin said the "underlying reason" was the "historic nature" of Brexit and the length of time the UK had been a member of the EU.
"Inevitably, there will be a long period of adjustment and change," he said.
Mr Martin added: "There's now a need to turn the corner in the relationship. And by that I mean the macro geopolitical issues are such that we need the European Union and United Kingdom in alignment, working together on the big issues that affect the globe.
"We saw great evidence of that in Cop26 - which I thought was excellently organised and hosted by the British government - and I believe the partnership between the United States, European Union and the UK Government on climate change is the direction of travel.
"That's the spirit that should inform our approach in terms of a sustained constructive relationship between Europe and UK into the future.
"I believe the first immediate step in that direction is a resolution of the issues pertaining to the operation of the protocol and, therefore, we should all direct our energies now over the next while to comprehensively, and in an enduring way, resolving those issues.
" I think if we can do that I think we can then develop that stronger, proactive, constructive relationship which is urgently required, in my view, given the way the world is going, between the European Union and the United Kingdom."
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The British-Irish Council, which is being held in Wales, brought together political leaders in Ireland and Britain to discuss Covid-19 as well as the Brexit issues.
The council also includes leaders from the devolved administrations of the UK, the Channel Islands and the Isle of Man.
As Brexit has done away with the regular meetings of Irish and British politicians and officials in Brussels, fora like the British-Irish Council become more important for keeping open political communications.
The current Brexit talks on the Northern Ireland Protocol will affect not just Northern Ireland, but Scotland and Wales too.
Their political leaders joined those from the Channel Islands and the Isle of Man around the table with the UK and Irish governments.