Green Party leader Trevor Sargent said he can see "no way" in which there would be sufficient numbers for a Fianna Fáil/Green government.
Mr Sargent also said it is "difficult to imagine that negotiations between the parties would succeed", but he warned the party cannot do a deal to go into government with Fine Gael if it is only offering "Fianna Fáil-lite".
Green Party leader Trevor Sargent
Speaking in Dublin to unveil the Green Party's election broadcast, Mr Sargent outlined what he said were his party's "deal makers" to go into government, outlining its demands to secure its support in any potential coalition with the larger parties.
Mr Sargent said "necessary changes" to clean up politics, to tackle climate change and to deliver first-class education services for children could not be provided by just Fianna Fáil and the Progressive Democrats or by a Fine Gael/Labour coalition.
"The people are telling us that they don't want a Fianna Fáil government back. This is clear from what we're hearing at the doors and it is clear from recent polls.
"The numbers don't stack up - the people want change. In any event Fianna Fáil would have their preferred option of doing a deal with Labour open to them," Mr Sargent said.
"It is clear that there is going to be a change of government. The Green Party has always said that we're ready for government. However, we cannot do a deal if Enda Kenny is only offering us 'Fianna Fáil-lite'."
Mr Sargent said the Green Party wanted the opportunity to participate in an alternative government.
He warned that basic services such as education could not be provided properly where cuts in income tax were also being promised. He said that was the "untenable" position of other parties.
The Green Party would invest €1 billion extra in education in each of the five years of the next government, he said.
"Our message to the voters is simple: the other parties' proposals in the areas of climate change, cleaning up politics and education are completely inadequate. You just won't get these changes with Fianna Fáil and the PDs.
"You won't get them either with Fine Gael and Labour on their own. Without the Green Party in government, you can expect more of the same old thinking, the same old approaches that don't work. The Green Party's proposals are innovative and prudent," Mr Sargent added.
"Other parties will have to come up to our mark when it comes to tackling the problem of climate change and rising energy costs. Failure to do so will leave our children in a world in crisis," he said.
Mr Sargent said the party would "make public transport work", delivering a "world class" public transport system. It would also "provide proper housing" and would break the link between the money of developers and the political parties.