Delivering a scathing assessment of the political logjam at Westminster, Simon Coveney said he was concerned Britain will "fail to get its act together over the summer".
His remarks came after the cross-party talks between Labour and the Conservatives collapsed on Friday without an agreement, and as Theresa May braces for yet another defeat to her Brexit plans.
If Ms May's Withdrawal Agreement Bill is rejected in two weeks' time, the prime minister will then set out a timetable of her departure from Downing Street - kicking off a Tory leadership contest.
But speaking to the Irish broadcaster RTE, Mr Coveney said: "The EU has said very clearly that the withdrawal agreement has been negotiated over two-and-a-half years, it was agreed with the British government and the British cabinet and it's not up for renegotiation, even if there is a new British prime minister.
"The personality might change but the facts don't," he said on Sunday, as he claimed Ms May was a "decent person" attempting to find the middle ground.
He went on: "For the EU and Ireland this has always been about the complexity of Brexit, trying to protect the EU, its integrity, its single market, its customs union, its members and also trying to respect the decision of British people.
"It's always been about that. For Britain in many ways it's been about party politics and personalities and many people seem to think that Britain would have got a much better deal if only they had a tougher prime minister.
"In my view that just is a fundamental misunderstanding of how the European Union operates."
Taking aim at MPs, he added: "There are many British politicians who don't, quite frankly, understand that or the complexity of politics in Northern Ireland and therefore they have tried to dumb this debate down into a simplistic argument whereby it's Britain versus the EU, as opposed to two friends tying to navigate through the complexity of a very, very difficult agreement."
Mr Coveney also warned the UK should not assume another Article 60 extension will be granted by the EU if a deal is not in place by the revised 31 October deadline.
Major changes and challenges as a result of the European elections next week, he claimed, could mean the EU being prepared to devote less focus on Brexit going forward. "That's my concern - that Britain will fail to get its act together over the summer," he said.
"There will be people like Nigel Farage and some within the Conservative Party who will be making the proposition that 'look, we have had enough of this, let's just leave on WTO [World Trade Organisation] terms without a deal' - in my view not fully understanding or not being honest about the full consequences of that for Britain and Ireland."
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