In a meeting lasting around 90 minutes, the Prime Minister updated her top team on the on-going discussions with the Unionists to secure an agreement propping up the Tory minority government.
Sinn Fein's Gerry Adams turned Mrs May's own slogan against her to brand it "a coalition of chaos", adding: "Any deal which undercuts in any way the process here or the Good Friday Agreement is one which has to be opposed".
Sinn Fein, the SDLP and Alliance have all made clear Northern Ireland Secretary James Brokenshire can not chair the ongoing process to restore powersharing at Stormont due to their perception he has a conflict of interest.
They are adamant the United Kingdom government can no longer cast itself as a neutral facilitator in the process, given the Prime Minister's likely deal with the DUP.
He said he was anxious principally that the planned agreement could hurt the Northern Irish peace process in which the British government plays an impartial role.
The Conservatives' ability to adhere to such a commitment if they are wedded to a parliamentary alliance with the DUP has been questioned.
May's Conservative Party needs the support of the DUP's 10 MPs in order to retain power in the House of Commons, after losing her majority in last weeks "snap" United Kingdom election.
Sinn Féin, he said, "won't interfere in British affairs".
The party was originally allowed to do so by then Prime Minister Tony Blair to encourage it into the political process but for the decade it has been the joint ruling party in Northern Ireland.
Sinn Fein, which won seven seats in the British parliament at last week's election but will maintain its policy of not taking them, said its leader in Northern Ireland, Michelle, O'Neill would repeat those concerns in London on Thursday.
Mrs May's authority has been severely diminished after a disastrous general election which saw her lose her Commons majority and a deal with the DUP looks vital for the continuation of Tory rule.
Northern Ireland's frontier with the Republic of Ireland will be the UK's only land border with the European Union after Brexit.
"The two issues - Northern Ireland and Brexit - might end up making the other more hard, in a vicious circle", Usherwood said.
Devolved government in Northern Ireland broke down in January.
"It's imperative that both governments recommit to the word, spirit and implementation of the Good Friday Agreement if there is to be any prospect of re-establishing the Executive".
"Our resolve is to see these institutions put in place on the basis they were founded upon as quickly as possible".