The appointment of Boris Johnson to Downing Street “quite rightly raises enormous fears”, Fianna Fáil leader Micheál Martin told an audience in Glenties, Donegal on Tuesday evening.
Speaking at the MacGill Summer School, Mr Martin suggested Mr Johnson had failed to impress Irish officials and politicians in his most recent cabinet posting.
Johnson beat Jeremy Hunt comfortably, winning 92,153 votes to his rival’s 46,656.
The former London mayor takes over from Theresa May later today (Weds).
Speaking to reporters in west Donegal before the event, Mr Martin said that when he met Mr Johnson last year “there was a sense that he was glossing over the realities of the potential negative of Brexit on the Good Friday Agreement… he seemed to push them to one side as if they weren’t an issue.”
He continued: “It is clear that he is single-minded in his ambition, but I don’t think anyone can credibly say that he has thought through how to promote prosperity and reconciliation in Northern Ireland.
“He has consistently, in my view, understated the impact of Brexit on North/South relations and on the island of Ireland.
“So I think he needs to bring himself up to date very quickly in terms of what is required actually to sustain the sets of relationships on this island.
“It took a lot of work to get there and I think he perhaps to date, maybe for reasons that suit his purpose in terms of Brexit, but I think he’s understated the impact of Brexit potentially on that agreement, but more importantly on the sets of relationships that underpin that agreement.”
However, Martin said he held hope that Johnson had the skills to adapt to the top British job.
“People would say he was an effective mayor of London. But to get elected as mayor of London, which actually for a conservative was a difficult proposition, but that he skillfully managed to do that, and then he moved to a liberal, environmentally conscious, a conservative middle ground that enabled him to win that election.
“I’m giving that as a potential hopeful sign, when he sees the realities of what’s before him, in terms of the engagement with the European Union, the realities of Brexit – there is no percentage in it for the UK in terms of a no-deal Brexit, or indeed for anybody, and that his own self-interest and the interests of his party and government will be bound up in arriving at a common sense appraisal of the challenge that Brexit presents to him.”