A Green Party TD says the programme for government with Fine Gael and Fianna Fáil does not go far enough to protect the lower paid.
Dublin South-West TD Francis Noel Duffy is opposed to the deal.
He told On The Record: "I think we've standing principles of social protection of our environment.
"The [carbon tax] dividend would be a problem and the housing strategy and the deficit from Fine Gael would be a serious problem.
"The [carbon tax] is progressive, therefore people on the lowest incomes would be paid - whether it's through social welfare or their tax credits.
"That's a major problem for me because the budget itself - if they're looking at reducing the deficit in 2021 - they're going to focus on various tax rises and that will include the carbon tax.
"For me, that's one of the major problems".
He said the proposed carbon tax "protects the earners at the higher level.
"Typically, anybody at the top or the bottom 70% of society will be protected, and the top 30 in this case now are being protected.
"And on top of that, then, this programme for government is giving a 3% tax cut to anyone earning over 100,000".
"I think each of the three parties compromised to make this deal, so nobody's coming away with the perfect deal".
Quoting academic research from the US, he said: "People have paid effectively to move away from fossil fuels... but at the same time, the money, the tax that's paid in is then paid back out either through social welfare monthly or you get tax credits.
"And essentially, the first 70% will actually get more than they pay out".
Deputy Duffy is also married to the Green Party Deputy leader Catherine Martin, who is in favour of the deal.
Asked if he felt like he was in a minority in the party, he said: "I think the party is diverse, both demographically and age wise.
"And there's opinions throughout.
"I like Depeche Mode and Catherine likes Bob Dylan - we've always had very robust conversations in this house about politics".
"Like any other couple we have our opinions on certain things and the same in politics - at this point we're disagreeing on this one.
"It's difficult but I respect her and she respects me".