The Fiscal Advisory Council has criticised the Government for increasing spending in this year's budget, saying it was "a deviation from prudent economic and budget management".
In its post-Budget assessment, the council said Budget 2016 complied with all fiscal rules, but said fiscal projections for the next five years were not realistic.
It said the Department of Finance's projections of fiscal space - the amount of money the next government will have to play with - are not realistic, as they do not fully take the cost of an ageing population or inflation into account.
By its calculations these pressures will use up virtually all of the available fiscal space, leaving the next government with little or no room for tax cuts or spending increases.
While Budget 2016 is judged to be consistent with fiscal rules and will keep the debt and deficit on a downward path, the council finds fault with budget 2015, in particular the extra €1.5 billion in spending announced on the eve of the budget.
Combined with an earlier spending increase in the Spring Statement, the council said spending this year will have grown by 4% - higher than planned spending increases in any of the so-called Celtic Tiger years - and was funded by an unexpected increase in corporation tax, which the council says is a volatile revenue source.
It said this has worrying echoes of past fiscal policy errors.
The Chairman of the Fiscal Advisory Council has described the increase of €1.5bn in spending as "inappropriate".
Speaking on RTÉ's Morning Ireland, Professor John McHale said the decision is a deviation of the prudence the government had exercised previously.
Mr McHale said using revenue windfalls to finance permanent increases in spending has unfortunate echoes of mistakes made in the past.
He said the council was not aware of the Government's decision to make the announcement.
And added that on the back of that they will be revising their memorandum of understanding with the Department of Finance to ensure they are made aware of such decisions.
Minister of State at the Department of Finance Simon Harris has said the Government is not going back to mistakes "repeatedly made by Fianna Fáil" while in government.
Speaking on RTÉ's Today with Sean O'Rourke, Mr Harris said the Government will continue on the path of prudent budgeting and that Budget 2016 is fully compliant with fiscal rules.
He said the Government has taken account of demographic pressure funding and that money has been set aside up until 2021 for such matters.
He added that the Government has a duty to get people back to work and that many public services require investment and it has to make a judgement call on these and get the balance right, in terms of how and where to spend money.
