The prime minister has faced opposition from many within her own party as well as the Democratic Unionists over the draft deal.
Ms May faced a tumultuous week that saw the departure of ministers including Dominic Raab, the Brexit secretary, and Esther McVey, the work and pensions secretary, and the launch of a high-profile insurrection on the back benches to remove her from office.
When the deal is brought before parliament, the vote is expected to be tight.
However, Ms McDonald said her party will not “ride in on a white charger” to save the day.
Sinn Fein MPs have not taken their seats at Westminster for more than a century in an abstentionist policy.
Ireland‘s deputy premier Simon Coveney has urged Sinn Fein to take its seats to vote on the deal, adding that if the Westminster ballot is lost by two or three votes then “people will rightly ask some pretty serious questions” of Sinn Fein.
Ms McDonald accused Mr Coveney of “playing to the gallery” during his Fine Gael party conference, which is taking place in Dublin this weekend.
“The tanaiste is playing to the gallery, he knows full well that Sinn Fein has been extremely active in defending Irish interests at every level,” she said.
“He knows Sinn Fein has an abstentionist policy, he knows full well that the British parliamentarians have to arrive at their decision, he knows that Sinn Fein cannot ride in on its white charger and make all of this go away.
“The tanaiste has urged measure in the comments that people make in terms of seeing this deal over the line. I think he needs to practise what he preaches.”
Ms McDonald also urged Ireland’s premier Leo Varadkar to be cautious as he addresses his party conference.
“I think the taoiseach and tanaiste as they speak to their party conference this weekend need to bear in mind that Brexit is for keeps, it is not done yet, we are still in the middle of a very, very dangerous scenario for this entire country,” she said.
“They need to lead from the front, not be speculators on the sidelines, not to be harbingers of doom, but to act responsibly, to lead and to ensure that whatever happens there will be no hardening of the border, that our peace agreements will be protected, that our economy will be protected and that citizens’ rights will be underwritten.”
Ms McDonald spoke to the media in Dublin on Saturday morning as she attended a meeting of Sinn Fein’s leadership over Brexit and its ramifications for Ireland.
The Democratic Unionists have indicated they intend to vote against the withdrawal deal when it comes before parliament, while Independent North Down MP Lady Sylvia Hermon said she is considering which way she will vote, adding she wants assurances from the prime minister.
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