Fianna Fáil has criticised Sinn Féin for refusing to take its Westminster seats for crucial Brexit votes.
Calling on Sinn Féin to take up their House of Commons seats, Fianna Fáil Brexit spokesperson Lisa Chambers said Mary-Lou McDonald's party have failed to recognise the extraordinary times we are in.
Ms Chambers warned that a Brexit deal is still not certain and said Theresa May was in the same position but failed to get her agreement past the House of Commons.
She added that if a deal is hammered out in Brussels in the coming days, it will still have to be passed in the UK, which could come down to just a handful of votes.
"If the votes come down to a situation where there are literally a handful of votes in ensuring that we don't have a no-deal Brexit and if Sinn Féin can stop that happening [they should].
"These are extraordinary times. I understand the abstentionist policy, but these are not ordinary political times. I think Sinn Féin has failed to recognise that," she said.
I think Sinn Féin needs to try and get back to work. They haven't been in work in Northern Ireland.
"They don't go to work in Westminster, and they've been quite ruthless in the advancing of their own agenda, using Brexit to push a border poll at a time when we know it's dangerous."
While she said there appears to be some progress between UK and EU negotiators, Ms Chambers warned that significant gaps remain and it is "quite likely" that talks will now go beyond Thursday's meeting of EU leaders and into next week.
"I think that the the megaphone diplomacy, to some extent, seems to have abated. We appear to have moved into more mature section of the negotiations.
"I hope that it's not too late to get a deal done."
On a potential vote on a final deal, Ms Chambers said: "It is important to remember that Theresa May got this far. She had a deal done, she had her cabinet behind her albeit with some resignations, and she couldn't get it through the House of Commons.
"And so this week is crucial. The mood music is still somewhat positive but there is a significant degree of caution from all sides."
But responding to Ms Chambers' comments, Sinn Féin MP for Foyle Elisha McCallion said her party would not be talking lectures from Fianna Fáil.
“The nationalist people of the north have turned their back on Westminster.
“The British Parliament has no regard and never had any regard for the interests of the people of Ireland.
“It is attempting to drag the people of the north out of the EU against their democratic wishes.
"Sinn Féin will take no lectures from Fianna Fáil who have abstained from all politics in the north for almost a century."
- Tuesday, October 8: Tánaiste Simon Coveney meets the EU's chief Brexit negotiator Michel Barnier in Brussels.
- Thursday, October 10: Taoiseach Leo Varadkar and British prime minister Boris Johnson hold a three hour private meeting in Cheshire, England. Afterwards, they release a rare joint statement saying there is a "pathway" to a deal.
- Friday, October 11: Leaks confirm this "pathway" has been trodden on before, and has an uncanny resemblance to Mr Johnson's predecessor Theresa May's 2017 deal. Under the proposals, Northern Ireland would leave the EU with the UK but would continue implementing EU customs rules, creating an Irish sea EU border and preventing a hard Irish border. Stormont could also be given a "consent" vote.
- Sunday, October 13: EU chief Brexit negotiator Michel Barnier says the Northern Ireland plan may be unworkable. The DUP rejects the existing deal.
- Monday, October 14: Intense EU-UK talks continue during the EU general affairs council and foreign affairs council in Luxembourg, with informed speculation indicating an emergency EU summit will be needed next week as a deal is unlikely this week.
- Tuesday, October 15: Mr Coveney and Mr Barnier mirror the deadline concerns in the morning. However, by late afternoon, sources suggest a deal could be struck early on Wednesday.
- Tomorrow, Wednesday October 16: Mr Barnier will brief EU member state ambassadors on whether a deal can be agreed at this week's EU summit.
- Thursday, October 17-Friday, October 18: EU leaders including Mr Varadkar and Mr Johnson will attend the latest Brussels EU summit. If a deal is ready, they will consider signing off on it. If a deal is not ready, they will discuss a potential emergency EU summit and whether to allow a "technical extension" of a few days.
- Saturday, October 19: Mr Johnson will attend a potential weekend House of Commons sitting to sign off on a deal. If he cannot produce a deal, he will be legally obliged to seek an extension to the October 31 Brexit deadline - provided he complies with British law.