You can never have too much of a good thing, or so the saying goes.

Except, it seems, if that good thing is corporation tax.

This week questions were once again raised about Ireland's growing reliance on the taxes flowing from the profits of businesses operating here, particularly large foreign ones.

First by the Departments of Finance and Public Expenditure and Reform in the Summer Economic Statement (SES). They warned the Government must be conscious of the "very real possibility" of a decline in corporation tax receipts, which now make up a substantial and growing part of the public purse.

Commenting on the SES, the Irish Fiscal Advisory Council (IFAC) said strong corporation tax receipts "should not be relied on to fund permanent spending increases".

That came a little over a month after IFAC warned it is now "urgent" for the Government to reduce its dependence on corporation tax.

The third note of caution came from the Central Bank, which in its summer quarterly bulletin described the current high levels of corporation tax as "the main risk to our public finances".

The red flags have once again reignited a debate about whether or not we are now over dependent on what could prove a volatile revenue stream, in a way similar to our addiction to the proceeds of property related taxes during the Celtic Tiger.

Michael McGrath and Paschal Donohoe publishing the Summer Economic Statement

Just how dependent on corporation tax are we?

The simple answer is pretty dependent.

Last year €15.3 billion was taken in by Revenue through corporation profit tax – compared to a decade ago when the comparable figure was just €3.5 billion.

In fact, so far this year the exchequer has taken in €3 billion more in corporation tax than it did in the same period last year - a 53% increase - which if maintained for the second half will push the take to around €20 billion.

Consequently, these receipts now account for nearly €1 in every €4 collected.

And if that weren't concerning enough, around half of these receipts are sourced from just ten large firms.

"To put it another way, this means that €1 in every €8 collected by the State comes from an exceptionally small number of firms, a concentration risk which represents a clear vulnerability for the public finances", the Summer Economic Statement said.

Where is all this extra tax coming from?

Figuring out exactly what is driving the bonanza in corporation tax isn't simple, but experts say there seem to be a number of factors at play.

In a sense, Ireland has become a victim of its own success, in attracting so many of the big US multinational players here in areas such as big pharma, medtech, IT and more.

They as a group have large and (in many cases) growing operations here, which have enjoyed particular success in recent years, meaning they are paying a lot of corporation tax on what are their high profits.

Add to that the changes to the US tax code in recent years which have driven profits Ireland's direction.

"The US companies were doing all that stuff with Bermuda and the Cayman Islands and the US went to eliminate and shut down that in the hope that the companies would write big cheques to the US," said Seamus Coffey, economist and lecturer at University College Cork.

"Well one consequence is actually they are writing big cheques to Ireland."

The OECD in Paris

There are also the effects of the OECD's Base Erosion and Profit Shifting process, which began in 2013, and eventually led to changes to how and where many companies pay tax.

"The central thesis of the OECD is to get companies away from tax havens where they have no substance and to put the profits where they have substance," said Feargal O'Rourke, Managing Partner of PwC Ireland.

"And we are now seeing that come home to roost because all US multinationals are now restructured and some have brought their intellectual property back to the US ... some have brought it onshore here, some have brought it onshore elsewhere."

So not only are many of the almost 1,700 multinationals operating here making big profits out of their Irish operations, but a higher element of those profits is now being recognised and taxed in this jurisdiction.

"It's a combination of good times and restructuring," Mr O'Rourke said.

A third factor is that within our corporation tax-payer profile, there is a heavy skew towards those large multinationals, like Apple for example, which by its own admission pays more tax in Ireland than any other individual or organisation.

Coupled together, these are the principal reasons why our corporation tax take has ballooned in recent years.

Why might the windfall end and how likely is that?

Predicting what will happen with corporation tax is not easy.

"If it is driven by external factors, well then you just don't know how reliable it is," Seamus Coffey said.

But broadly speaking there are a number of ways in which corporation tax revenue could take a sudden and severe dip in the short- to medium-term.

First is the risk of a global economic downturn driven by the war in Ukraine, the supply chain difficulties, the energy crisis, inflationary pressure, rising interest rates and all the other issues going on right now.

That could hit the profits of multinationals and force them to retrench, undermining the corporate tax take.

This scenario seems not unlikely, given the warnings this week from IMF chief Kristalina Georgieva that she could not rule out the possibility of a global recession next year, although quite how severe it might be is uncertain.

The second worry would be if one or more of the big top ten or 20 multinationals here were to decide to shift operations or assets elsewhere, moving the profit and taxes with them.

"Most of them have very strong and long and deep presences here," said Feargal O'Rourke.

"But it's just the law of large numbers. If one of them decides that over the coming years Ireland needs to be less important in their overall global structure, well then the Irish tax take will disproportionately fall."

A third possibility is further changes to the US tax code that entice American multinationals to move profits back there instead - but that seems unlikely.

"The Biden administration came in very vociferously on corporate tax at the start of their term, but it has slipped completely off the agenda," Mr Coffey said.

A fourth trigger could be the implementation of the latest OECD global tax agreement which will force Ireland's corporation tax rate to increase to 15%, but may mean less tax is paid here by multinationals based here that sell across Europe.

The Department of Finance has concluded that the cost of this could be €2bn a year, but in reality until it happens nobody can be too sure.

If it doesn't get implemented, which continues to be a real possibility, then there's a broader risk of a trade war, where Europe starts implementing digital taxes and the US retaliates - a sequence of events which could also have an impact on corporation taxes.

What Mr O'Rourke thinks we should be less concerned about though is the prospect of assets like intellectual property, that was moved on shore here as a result of the BEPS process, being shifted elsewhere in the near future.

"Moving intellectual property is not something you do every year or every couple of years," he said.

"This was a fairly seismic change, once the Trump tax cuts and jobs act came in 2017 and then the OECD proposals, it is kind of almost a once in a generational sort of structural change."

"There is not a tax director I know of in these US companies who is not saying great, that’s done, I don’t have to think about that for the rest of my career, it will be my successor's job."

Balancing the Budget would become more difficult

What would it mean for the Budget arithmetic if we didn't have all this corporation tax?

In the Summer Economic Statement, the departments set out that under a hypothetical scenario in which corporation tax receipts had remained unchanged at pre-pandemic levels, deficits would have been in prospect for this year and next.

They calculate the scale of those deficits to be somewhere in the region of -1.5% of GDP and -1.25% of GDP, respectively.

In monetary terms, that translates to a shortfall of up to €7bn.

While the Central Bank and IFAC's modelling suggests as much as €8bn of corporation tax last year was unexpected or excess, given the actual growth in modified national income.

If some or all of that were to be taken out of the overall budget for public spending, it would have enormous consequences.

What can and should we be doing about it all then?

The clear message from all experts is that we need to do two things.

The first is ensure that windfall corporation tax receipts do not get baked into current spending in any way, so that if they were to vanish the knock-on consequences would not hit day-to-day services.

The Minister for Finance has argued that is what is already happening, with last and this year's excess corporation tax spent on one-off and capital spending.

The other thing we need to do, stressed by the IDA last week, is to ensure Ireland's economic and social environment remains conducive to doing business, so that the big multinationals and others want to continue investing in and staying here.