Labour leader Alan Kelly says he believes the country would have a new government by now if his party had won as many seats as the Greens did in February's election.
The Greens - who have 12 seats in the current Dáil, making them the fourth largest party - are currently involved in negotiations with Fianna Fáil and Fine Gael on a programme for government.
Both Labour and the Social Democrats - who each have six seats - were also invited to join the negotiations.
With the Social Democrats having now firmly ruled out that prospect, Labour is also set to formally reject the prospect of talks tomorrow.
Deputy Kelly told The Hard Shoulder his party's TDs and senators will meet tomorrow to discuss the issue, but the outcome seems certain.
He explained: “Very simply… if we got what the Green Party did in relation to seat numbers, I think you’d have a government in Ireland by now. But we only got six seats unfortunately.
“We have engaged in a process, and we will be writing back to Fianna Fáil and Fine Gael tomorrow after our parliamentary party meeting.
"We have serious concerns as regards to what they’re saying about taxation."
He said the two larger parties also "haven’t responded to a whole range of other issues" that his party asked about.
The Labour leader noted: "Our view - and I’ll be signing off on this with my parliamentary party tomorrow - is we’ll write back and say the process with the Green Party should conclude, and any three of the four parties who won the most seats can form a government.
“Unless there were significant changes, we wouldn’t entertain a government that wasn’t prepared to look at how we’re going to ensure that we could raise revenues to ensure essential services over the coming years."
He insisted that his party is always happy to talk to others about the prospect of forming a government.
However, he argued: "We’ve always gone into government when the country was on its knees, and we always got punished for doing so because we put the country first.
“We’ve only six seats - while we’ll always talk to people, I envisage this government will be formed with the Green Party."