Sinn Féin are insisting an internal memo for Michael Noonan shows it is unfair and unaffordable to abolish the Universal Social Charge.
That is despite claims from the Department of Finance that the memo has been made redundant by the new government.
Pearse Doherty says an internal memo, outlining the extra taxes needed if USC is abolished, proves the charge is too expensive to lose.
Sinn Féin's Finance spokesperson said information he got from the Department of Finance under Freedom of Informationshows that the plans to phase out the USC have been slated by department officials as "regressive", "base narrowing" and would have "limited benefits".
The information outlined a range of alternative revenue sources including:
A 600% increase in property tax,
A 5% increase in both income tax bands,
Increasing Corporation Tax to 19.5%,
A 5% VAT rate on children shoes and other previously 0% rated items.
The department disagrees and says the charge will be phased out gradually over five years.
However, Deputy Doherty says the briefing paper specifically says how unfair that would be.
He said: "No, the paper actually deals with the phasing out of the USC over a period of time.
"There is a part in the paper that deals with that, and indeed it's in that part where the Department of Finance say that this will have a benefit to the wealthiest in society with little or no benefit to those who are on very low incomes.
"So the department has assessed exactly what the Government are planning to do and have come up with a judgement in relation to it."