The Economic and Social Research Institute says the economy will continue to grow next year, even in the event of a no-deal Brexit.
However, such an outcome would cost at least €4bn in lost output compared with the orderly Brexit envisaged by the Withdrawal Agreement.
The ESRI forecasts the economy growing next year by about 4%.
That assumes the UK will have an orderly Brexit based on the Withdrawal Agreement, which includes a two-year transition period in which effectively nothing changes.
But other forms of Brexit would cost economic growth here.
In the so-called Norway scenario of the UK joining the European Economic Area, Irish growth would fall to 3% of GDP, while in the WTO, or no-deal scenario, growth would be lower again at 2.5%.
In cash terms, that is about €4.5bn of lost growth next year, meaning the Government would probably have to borrow money to meet its budget plans.
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The ESRI says 2018 will turn out to be another "exceptional" year for economic performance in Ireland, with GDP growth expected to be 8.2%, and employment growth of 3%.
Even allowing for the effects of multi-national companies on Ireland’s small open economy, the ESRI says underlying economic growth will be around 4.5% to 5% this year. It says average unemployment will be 5.7%.
Next year its baseline assumption is for economic growth to be 4.2%, with average unemployment of 5.1%
However, it says the economy faces an unprecedented degree of uncertainty in 2019; the outcome of the Brexit process, combined with the possibility of increased international trade tensions, could have significant implications for the economy's performance in the New Year.
Budget 2019 saw a significant increase in Government expenditure, particularly on capital projects.
As a result, it is now likely that there will be a deficit in the general Government balance in 2019, whereas a surplus had looked a possibility before the budget.
No-deal Brexit could cost Irish economy €4 billion pic.twitter.com/4PjVuWANXj— RTÉ Business (@RTEbusiness) December 12, 2018
While most taxation headings are witnessing sizeable increases in 2018, a key feature of the public finances in the present year is the significant growth in corporation tax receipts.
As a substantial portion of this growth is due to a small number of companies, and so potentially vulnerable to a reversal, it says it is "imperative that policymakers do not base expenditure on potentially volatile revenues going forward".
The ESRI notes that, while income tax and VAT receipts have gone up by around 6% each, Corporation Tax receipts have increased by 22% this year, following 13% growth in 2017. Average Corporation Tax revenue growth has been 9%.
If Corporation Tax revenues were to grow by this rate this year and next, the size of the projected Government deficits for each year would double - highlighting just how sensitive the public finances are to changes in corporate profitability and the taxes it produces.
This is why the ESRI warns the Government to be careful, saying such high growth rates imply a large windfall element in recent Corporation Tax receipts.
Its assessment of Budget 2019's increase in spending is that it was more expansionary than the ESRI would have liked, but it was not imprudent.
It says the overall budget package does increase the risk of overheating in the economy, though it says the increased spending on social housing provision is to be welcomed.