Around half of the projected fiscal space for Budget 2018 has already been used to pay for the full year costs of tax cuts and spending rises announced in Budget 2017.

According to the Department of Finance, there is an estimated net fiscal space of €1.2 billion available for tax cuts and spending increases in Budget 2018.

But in a written reply to Fianna Fáil Finance spokesman Michael McGrath, the Minster for Finance Michael Noonan says the carry over effect of tax measures announced last month for Budget 2017 will absorb €172.5 million of net fiscal space.

Meanwhile, the Department of Public Expenditure and Reform estimates the carry-over of spending increases from Budget 2017 will impact the 2018 fiscal space by approximately half a billion Euro. 

That would leave just over half a billion Euro of unallocated fiscal space for budget 2018.

Mr McGrath says that slowing levels of economic growth may reduce the €1.2bn figure further, and that new public service pay deals could leave little or nothing for new tax cuts or spending increases.

By achieving a balanced budget by the end of 2018, the fiscal space estimated to be available for Budget 2019 will be substantially bigger than recent budgets - around €2.7bn, a figure that is projected to be replicated for budgets 2020 and 2021.

That would give the Government some scope to use timing effects to increase the full year costs of the budget 2018 package.

The existing public sector pay increases are provided for in the budgets for 2017 and 2018 (€300m each year) but not beyond, so a large slice of the extra projected fiscal space may be targeted by public sector unions.

The Government has already announced its own plans to siphon off a portion of the fiscal space from 2019 into a "rainy day fund" - about a billion each year - to provide a shock-absorber for future economic problems, giving it some capacity to spend money in support of the economy in a downturn. 

Sticking to that plan leaves about €1.7bn a year in available fiscal space - before any new public pay deal. This compares with a budget package of €1.3bn in budget 2017.

All of which will likely leave a fairly modest sum available to fund any further tax relief or spending increases in the years to 2021. 

And that assumes growth holds up as forecast, it has already been marked down after the Brexit referendum, and the projections for fiscal space for 2019-2021 have been cut by €300m each year, compared with the estimates in the Summer Economic Statement.