The British government has agreed that it will introduce an Irish language act in Northern Ireland in October if the Stormont Assembly does not do so by September.
At 12:22am on Thursday, Ms McDonald tweeted: "The British Govt has tonight agreed to legislate for Acht Gaeilge".
"This is the only way to break the cycle of DUP obstruction of rights".
The DUP appears to be in internal disarray after a significant majority of its elected representatives earlier opposed Mr Poots' decision to nominate a Stormont First Minister.
"We have monumental challenges ahead that will require unity of objective and urgency".
"This would be better done in the Assembly as agreed past year but it is welcome that the Secretary of State has given an assurance these positive measures agreed in support of linguistic diversity and cultural expression in Northern Ireland are now to be put in place through legislation in Westminster if necessary", Mr Coveney said.
McDonald added that the agreement with the United Kingdom government marked a "special moment" for Irish-language speakers.
DUP leader Edwin Poots and First Minister designate Paul Givan left a meeting with Mr Lewis at around 8.45pm.
The joint nature of the office Mrs Foster shared with Ms O'Neill in the latter's first term as deputy First Minister meant Ms O'Neill was automatically removed from the post on Monday and had to be nominated for the role again within those seven days.
"We look forward to working with the First and deputy First Minister, and all the parties in the Northern Ireland Executive, to the mutual benefit of people North and South, including at the next plenary meeting of the North South Ministerial Council which is planned for Friday in Armagh".