A report which suggested an annual bill of €20 billion for 20 years to fund a united Ireland has been described as a "worst case scenario" by a Sinn Féin TD.
The report by the Institute of International and European Affairs said the significant expense would require an increase in taxation of around 25%, and/or a significant reduction in expenditure.
The costs are based on pre-pandemic 2019 figures and would be higher based on today's figures.
However, Donegal Sinn Féin TD Pádraig Mac Lochlainn said the suggested costs "would be heavily disputed by a range of other professors and economic analyists".
Speaking on RTÉ's Today with Claire Byrne, he said the report assumes there would be no economic growth in Northern Ireland, while there has been substantial growth in the all-Ireland economy since Brexit.
Deputy Mac Lochlainn said "there's a real opportunity for economic growth," because of Northern Ireland's "unique" open border to the Republic, and despite Brexit, has access to both the EU and the British economies.
He disputed the methodology of today's report, calling it a "static analysis," and argued the actual cost is less than the headline figure of €20bn.
Deputy Mac Lochlainn said the basis of the report accepts an annual "subvention" from the UK government to run Northern Ireland of around £9.5bn (€11bn), "at face value".
However he said: "Professor John Doyle of DCU in recent times has written that the actual subvention is closer to about €2.4bn."
'Wildly varying" assumptions
Any reports on the economic costs of a united Ireland depend on a number of "wildly varying" assumptions, an economics professor with the ESRI has said.
Professor Seamus McGuinness, Research Area Co-ordinator for Labour Market Research at the ESRI, said subvention, the amount the Irish Government would have to pay to help support a united Ireland, is difficult to speculate on.
"We've seen estimates from the perspective of analysts varying from around Stg£3 billion to up to £11bn in today's analysis," he said.
Prof McGuinness said the underlying causes of subvention are the lower than average productivity in Northern Ireland but added: "We need to recognise that productivity is not something set in stone, it's something that can respond to policy and investment.
"And therefore can be reversed. And therefore, subvention costs can be either reduced or eliminated over time given the right policy framework."
He said any transition to constitutional change would involve a transition period.
"During that transition period, really, we want to have pre-planning for growth," he said, adding that applied subvention would react to investments over time.
When it comes to what the British government might pay, he said there was an argument that pensions liabilities could remain a British liability going forward.
He said there was also an argument that tax-take would go up, if the Irish tax code was applied in Northern Ireland.
However, he said, all these arguments missed the point that any planning for constitutional change would need to focus on the underlying causes of substantial low productivity in Northern Ireland, where there is now a 40% gap.
This would require policy simulations and proper planning.
"So at the point of a border poll, people in Northern Ireland have a clear vision of what reunification will represent and the implications of that for them going forward. And that planning can only be done by the Irish Government.
"And we can't really repeat the steps of Brexit by going into into major constitutional referendum without proper planning. And my view is that planning process needs to begin in earnest as soon as possible."
The Institute of International and European Affairs study showed that unification would add around 5% of modified Gross National Income (GNI) to the Irish Government's deficit.
If rerating of welfare payments and public sector pay rates in Northern Ireland (to align them with those in Ireland) were included, the cost would be almost 10% of modified GNI.
This would add a quarter to public expenditure in Ireland (total Government expenditure in Ireland currently amounts to around 40% of GNI), while producing a very limited increase in revenue.
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To deal with the resulting deficit, which would be likely to persist for many years after unification, there would have to be a dramatic increase in taxation and/or a major reduction in expenditure south of the border.
"Even though Ireland has a much higher national income, funding the needs of the people of Northern Ireland in a united Ireland would put huge financial pressure on the people of Ireland, resulting in an immediate major reduction in their living standards," report co-author and Co-Chair of the Institute of International and European Affairs Economists Group Professor John FitzGerald said.
Speaking on RTÉ's Morning Ireland, Prof FitzGerald compared the sum of €20 billion a year to "a third of a bank bailout every year".
"The solution is if Northern Ireland dramatically changes how they run their economy, in particular in the educational sphere, which would mean that the gap between the north and the south would narrow and also that their revenue would rise substantially more.
"Looking for somebody else to pay for unification is not going to happen. If it happens, we've got to pay for it ourselves."
The authors argue that the cost of unification could be substantially reduced if Northern Ireland made major changes in its economy in order to raise its productivity.
If Northern Ireland chose to remain in the UK indefinitely, by reforming its economy it would greatly enhance its economic position within the UK.
This would result in a substantial improvement in its relative standard of living, the authors said.
Reform would reduce the Northern Irish deficit and, therefore, also substantially cut the costs associated with potential unification.
Under the most favourable circumstances, it is likely to be at least two decades before the productivity gap could be substantially narrowed, report authors said.
Speaking on the same programme, Minister of State for Trade, Promotion, Digital and Company Regulation Dara Calleary said he believes unification would be "a hugely good thing" for the country.
Mr Calleary said there are huge benefits to all-island health and education systems, adding that he had not yet examined the study in detail.
The report would ideally be brought before an Oireachtas committee, he said.
