Business group Ibec has challenged the Government's method of calculating the amount of money it will have for additional budget spending - the so-called Fiscal Space.

Ibec says a Government decision to disregard 2015 economic data leads to an underestimation of available Fiscal Space of between €4bn and €7bn over the next four years.

2015 was the year of Leprechaun economics, when the country recorded an official growth rate of 26% of GDP, thanks to a change in statistical methods and an influx of multinational investment.

But the 26% figure was such an outlier that government economists decided to disregard the 2015 figure in calculations for Fiscal Space out to 2021.

They used instead an average of 2014 and 2016 growth, arguing this gave a more realistic result.

This decision was backed by the fiscal council.

Ibec says the complexity of the fiscal rules means the political implications of that decision have received very little public debate.

Ibec says that decision means the Government is underestimating the amount of Fiscal Space it has available by about €4bn over the next four budgets.

With base effects, that could rise to a cumulative increase of €7.2bn over and above the money indicated in the Summer Economic Statement.

Ibec says some of this additional Fiscal Space could be used for investment in one-off capital programmes such as roads or ports, and another part earmarked for a contingency fund to deal with economic shocks from Brexit.

It claims the additional money would be consistent with the fiscal rules, and a balanced current budget.

The additional Fiscal Space, according to Ibec calculations, would amount to €400m for next year;s budget and €300m in 2019, before rising to €1.4bn and €1.9bn in 2020 and 2021.

Under the fiscal rules, spending growth is limited by an "expenditure benchmark", which is based on an estimation of the potential output growth of the economy.

This is derived from a ten-year average (forward and backward in time) of potential growth estimates.

By using an average of 2014 and 2016 growth, rather than the recorded GDP figure for 2015, the ten-year average number is lower than it would have been if the actual 2015 figure was used.

Ibec also says the Government is inconsistent in its treatment of the surge in corporation tax revenues in 2015, which were not disregarded, but which were spent.

It says a more consistent approach would be for the Government to use the greater Fiscal Space underpinned by the extra corporation tax receipts for one-off public capital projects.

This, it says, has two advantages.

Firstly, it would allow increased levels of capital spending while still running a current budget that is balanced or in surplus.

Secondly, it would avoid building in the future volatility of corporation tax receipts into the base of current spending, which it says is the approach taken by the Government to date.