Sinn Féin, the SDLP, Alliance and the Green Party all wanted to hold joint discussions with her.
A spokesperson for Downing Street said a meeting was not possible.
But the idea is controversial among May's eurosceptic Conservative MPs, who fear it is simply a way of delaying Britain's exit.
European Union leaders must now decide whether to hold an emergency summit next month, despite warnings from leaders including French president Emmanuel Macron that no further meeting could take place without progress on the Irish border.
It was a marked change of tone since May's Brexit minister Dominic Raab left Brussels on Sunday after the more than year-long Brexit talks broke down over the border issue.
On Twitter, SDLP leader Colum Eastwood said the prime minister's decision was "not surprising".
"And we should be clear that, as for now, not enough progress has been made", he said.
Asked if the whole deal could founder over the border issue, he replied: "My answer is yes".
Speaking on the third day of the EU summit meeting in Brussels, Varadkar said without an Irish backstop any withdrawal treaty may fail to be ratified by the European Parliament.
"The Secretary of State for Northern Ireland met all five main parties on Monday 8 October and will meet them again soon".
"And the European Parliament has been very strong in supporting Ireland and our need to have a backstop to avoid a hard border and, particularly when it comes to citizens' rights, the European Parliament has been a very strong ally too".
Seeking to unblock the stalled divorce talks May said the initial 21-month transition could be extended by "a matter of months".More news: Is Donald Trump undermining the Fed's independence?
'The current position, as I understand it from Michael Gove, is that we will be negotiating as an independent coastal state in December 2020'.
"We are not trying to scaremonger here, what we are trying to do here is protect a very precious peace and normality on the island of Ireland", he told BBC Radio Ulster.
Tusk says "what I feel today is that we are closer to the final solutions and the deal". The most notable was that of the Irish Taoiseach, Leo Varadkar, who brandished an Irish Times article on the 1972IRA bombing of a border customs post as he sought to stiffen the resolve of his political peers and highlight the risks of giving way on the border problem that has so dogged the negotiations.
Yesterday, she tried to convince the European Union heads of state and government of her plans.
On the Irish border issue Tajani said, "We want a flexible border and to preserve the Good Friday Agreement, but we also want to protect our agri-food sector, industry and health".
But the Northern Irish question still has to be overcome.
The senior source said the leaders agreed talks should continue but, for now, they are "not planning to organise an extraordinary summit on Brexit in November".
In her news conference Mrs May appeared to rehearse the arguments she will likely use to argue back against those criticisms - that the idea isn't new, that it wasn't requested by the United Kingdom, but that it is worth considering if it opens up the blockage of the Irish backstop.
The transition period is created to smooth the path to a future permanent relationship.
"I think where there is a will, there is a way", German Chancellor Angela Merkel is reported to have said, while it is thought the President of the EU Commission, Jean-Claude Juncker, told leaders May needs "help" to sell a deal back in Blighty.
Secondly, such an extension might reduce the chances of any backstop being needed in the first place because it would allow extra time for negotiations of a future trading relationship, which Mrs May has long suggested would make the backstop unnecessary.
The implementation of the transition period is contingent on a withdrawal agreement being found, however.