Irish premier Leo Varadkar has said everyone in Ireland and the UK should be afraid of a no-deal Brexit.
Mr Varadkar also rejected criticism that he was engaging in “project fear”-style rhetoric over a united Ireland after Brexit.
DUP leader Arlene Foster said the Taoiseach needed to “dial down the rhetoric”.
Last week Mr Varadkar said moderate unionists and nationalists could question being part of the UK if they were forced into a hard Brexit.
Mrs Foster accused him of behaving “crassly” towards victims of the Northern Ireland conflict in the past.
“He needs to dial down the rhetoric, he needs to recognise the mandate of the Prime Minister and he needs to engage. He needs to get engaged and he needs to find a way forward.”
But speaking in Kilkenny on Thursday, Mr Varadkar said: “In terms of engagement, I have never refused a meeting request from the DUP and never refused a phone call from Arlene either, and she has my number.
“I wouldn’t accept that criticism at all. What I would point out though that when it comes to negotiations on Brexit, they happen between the European Union, including Ireland on the one hand and the UK Government on the other.
“No political party is involved in these negotiations, they are inter-governmental by nature, and I have spoken to the new Prime Minister by phone and I have invited him to come to Dublin to talk about these matters some more without any preconditions, so that’s really an invitation for him to decide on.
“A no-deal Brexit would have very serious impacts on the economy, north and south, and on Britain. It could have security implications as well and it could have constitutional implications.
“It’s something that we have to prepare for nonetheless. It is something we should be afraid about.”
Earlier this week, Boris Johnson held a private dinner with senior DUP figures as he visited Northern Ireland for the first time as Prime Minister.
Asked whether he was worried about the Prime Minister’s ability to be impartial over Northern Ireland after the private dinner, Mr Varadkar said: “I think that remains to be seen.
“He’s only just started in the job and I think we need to give him a fair wind and a decent chance.
“But you know, we shouldn’t also ignore what’s there in the Good Friday Agreement, and I hear a lot of talk about the Good Friday Agreement in recent weeks and months and often wonder if some of the people who quote the Good Friday Agreement have actually read it.
“The Good Friday Agreement is very explicit that the sovereign government, the UK Government must be rigorously impartial in how it administers Northern Ireland, and we all need to respect the fact that the aspirations about unionist people and nationalist people are equal.”
He encouraged political leaders in the UK to read and “absorb” the Good Friday Agreement, adding that he re-read the 1998 document last week.
He added: “It really is a masterpiece in terms of agreement and really is so eloquent and still so relevant today, and I just would certainly encourage anyone holding high office in the United Kingdom to read it and absorb it.”