Sinn Fein TD Aengus O'Snodaigh said the party would give Labour a tough time if it joined Fine Gael on the government benches.
"The Labour Party in particular have sold themselves as a friend of the working people. If they go into bed with Fine Gael then they will be very, very strange bedfellows and we will see what policies they introduce," he said.
"And we will tackle them on the ground and in the Dail on every one of those policies."
Mary Lou McDonald, Sinn Fein vice president, again ruled out working with Fine Gael to form its own coalition pact.
"The people who voted for us and the mandate we have is very clearly a mandate to stand up against an administration, whoever is in it, that insists that the burden of the economic calamity would be put on the backs of people on low incomes," she said.
"The idea that you could cosy up to Fine Gael to do some kind of a deal... just wouldn't be a runner."
Ms McDonald said she expected party president Gerry Adams, who won a seat in Co Louth, to lead the party in the Dail.