Sinn Féin's negotiators have told other parties it wants to form a coalition government with Fianna Fáil in private talks held in recent days.
The party has signalled its preference for what is being dubbed a "left-led" government with Micheál Martin's party if it cannot form a minority left-wing government without Fine Gael or Fianna Fáil, according to TDs who have held discussions with Sinn Féin's negotiators.
Sinn Féin's position is likely to put further pressure on Mr Martin, who has continued to rule out any coalition with Mary Lou McDonald.
While he has the support of many in his parliamentary party, a number of Fianna Fáil TDs have privately told the Irish Independent in recent days this position may not be sustainable if a government is not formed.
Sinn Féin's negotiating team, which includes Pearse Doherty, Eoin Ó Broin, Louise O'Reilly and Matt Carthy, has held talks with the Green Party, Social Democrats, Rise-Solidarity-People Before Profit, and Independent TDs in recent days.
Green Party leader Eamon Ryan said Sinn Féin's preference is to include Fianna Fáil, the Greens and the SocDems in the next government.
People Before Profit TD Richard Boyd Barrett said that while his preference is for a left-wing minority government without either of the Civil War parties, Sinn Féin's negotiators had signalled a willingness to govern with Fianna Fáil.
"They were keeping open the option of deals with others, which I took to mean Fianna Fáil, given they are open to talking to them, but they did point out that Fianna Fáil don't want talk to them," he said.
Rise TD Paul Murphy said: "They are saying they are pursuing two options, minority government or what they call left-led government, which is coalition with Fianna Fáil.
"We're of course disappointed that they are considering putting Fianna Fáil back in power."
Sinn Féin TDs involved in the negotiations did not return calls last night.
The Irish Independent has also learned that Sinn Féin's negotiating team has appealed to other parties and Independent TDs, who they have met in recent days, to back Ms McDonald in the Dáil vote for Taoiseach taking place today.
The five Rise-Solidarity-People Before Profit TDs are expected to back Ms McDonald, giving her at least 42 Dáil votes which is likely to be more than either outgoing Taoiseach Leo Varadkar or Mr Martin.
However, she will not be elected Taoiseach as she will not command a Dáil majority.
Mr Martin last night appointed a negotiations steering group to meet with other parties which will report back to its front bench and parliamentary party.
It includes deputy leader Dara Calleary, children's spokeswoman Anne Rabbitte, finance spokesman Michael McGrath, public expenditure spokesman Barry Cowen, housing spokesman Darragh O'Brien, and education spokesman Thomas Byrne.
However, the party's justice spokesman, Jim O'Callaghan, who was involved in talks with Fine Gael on confidence and supply four years ago, is not involved in the delegation.
The Green Party will begin bilateral talks with other parties next week.
It is due to meet a delegation from Sinn Féin on Monday, Fianna Fáil on Wednesday, and Fine Gael towards the end of next week.
The talks will be facilitated by the National Economic and Social Council (NESC) secretariat.
The Fianna Fáil parliamentary party met last night in Leinster House but did not discuss in detail the ongoing government negotiations.