THERESA May's former chief of staff has claimed Taoiseach Leo Varadkar is "the real threat" to peace in the north.
Writing in the Daily Telegraph, Nick Timothy said Mr Varadkar had raised the chances of a hard border, post-Brexit.
He accused the Fine Gael leader of "jeopardising the peace process".
Mr Timothy was one of Mrs May's closest advisers but quit after being on the receiving end of heavy criticism for his key role in the Tory election campaign in 2017.
He wrote that Mr Varadkar had "backed himself into a corner".
"He doesn’t understand the Good Friday Agreement and he's jeopardising the peace process. He's risking a no-deal Brexit and a hard border on the island of Ireland," Mr Timothy said.
"He continues to defend his Brexit policy with a brave face but, like a poker player who has raised the stakes too high, he must surely feel his stomach churning as he contemplates the consequences of his gamble."
New UK Prime Minister Boris Johnson has clashed with Brussels over his call for the EU to drop its opposition to a new Brexit deal and return to the negotiating table.
He said he would work "flat out" to secure a new agreement.
But Brussels responded swiftly saying the existing Withdrawal Agreement was "the best and only" deal possible.
Mr Johnson said the British government was "turbocharging" preparations for a no-deal break on October 31 if the EU refused to engage.
Mr Timothy wrote that a no deal Brexit would be "catastrophic for Ireland" but said Mr Varadkar should have changed tack during discussions following the 2016 referendum.
"Varadkar therefore had a choice. Recognising our shared interests, he could have argued for a future UK/EU relationship that allowed Britain to leave the single market and customs union while still minimising friction in cross-border trade. He could have continued the work begun by his predecessor, Enda Kenny, to use policy and technology to avoid customs checks along the Northern Irish border.
"Instead, he gambled, and exhorted the EU to take the hardest line possible. Before the UK and EU could negotiate their future relationship, he insisted, the border question must be decided. Never mind that this was nonsensical and everybody knew the border could only be fixed in a future trade agreement.
"With Vardakar's connivance, Brussels weaponised the Northern Irish border - and with it the peace process - to lock the UK into a customs union and a colonial status in which we would have to follow EU laws."
Mr Timothy added that the backstop was supposed to stop a hard border.
"By making the Withdrawal Agreement unratifiable, it is making no-deal and a hard border more likely," he said.
Responding, the Republic's Minister of State Patrick O’Donovan said Mr Timothy had been gone from Downing Street for a long time and had not been party to the most recent discussions, even under Mrs May.
"Language is hugely important and language can be very emotive. Calmness is absolutely necessary," he told RTE's Claire Byrne on the News At One.
"The Good Friday Agreement is at the centre of the Irish government's position and at the centre of the EU position. The taoiseach's position is not at variance one bit from the EU 27."
Steve Peers, an academic and an expert on the EU, was among those who criticised Mr Timothy's article.
"Nick Timothy is responsible for the `red lines' that made the negotiations particularly difficult," he said.
