For those with a passing or healthy interest in tax and how the state is funded, Revenue's annual report published this week is a wealth of information.

From tobacco smuggling to Brexit and property tax to enforcing payment compliance, it provides a deep insight into how the collection system works.

But some of the most fascinating data was actually contained in a separate piece of research, published alongside the report.

The "Corporation Tax 2021 Payments and 2020 Returns" provide an interesting breakdown of how much the State's coffers were dramatically boosted by tax collected from business last year.

In total corporation tax receipts totalled €15.3 billion in 2021, up an enormous €3.5 billion from the €11.8 billion that was paid over a year earlier.

It meant that tax from firms big and small accounted for a very significant 22.6% of total tax receipts - making it the third largest tax head and a hugely important revenue stream for the exchequer.

That was even more so the case last year as we were in the grips of the Covid- pandemic, when some sources of tax were inherently unreliable because of the ever-changing impact of restrictions on the economy.

Since 2015 corporation tax has been steadily increasing its share of the overall total of tax receipts.

This has been the result of both strong growth in the Irish economy, particularly in the multinational sector, and changes to international tax rules which resulted in a bonanza for such firms operating here.

Much has been spoken and written about our growing reliance, or over-reliance as its seen by some, on this windfall of corporation tax receipts.

The big fear is that much of it may disappear as quickly as it has arrived in recent years.

But perhaps of even more concern is the reliance within that corporation tax head on a few very large companies.

The Revenue research showed that of the 66,400 companies that paid corporation tax last year, a mere 155 were responsible for paying 75% of the total take, or €11.6 billion.

Drilling down further you find that the top ten payers alone collectively paid 53% of the corporation tax total or some €8.176 billion.

That represents an increase from the 51% share that they contributed in 2020, the 40% in 2019 and the 32% in 2010.

Or looking at it another way, 12% of the total overall tax take of €67.5 billion last year came from just ten giant firms.

So not only are we growing ever more dependent on corporation tax, but we are also becoming increasingly reliant on a small pool of businesses for the bulk of it.

We don't know for sure who the top payers are, as Revenue never discloses the details of its clients.

But we do know, because CEO Tim Cook confirmed it in 2016, that Apple was Ireland's largest taxpayer back then.

And given that the latest accounts filed for its main Irish based operating subsidiary, Apple Operations International Limited, show it paid $8.5 billion in corporation tax in 2020 (not all of which was paid in Ireland) we can probably assume it remains among the biggest contributors.

As for the rest, we know from other filings that other large contributors likely to be in the top ten include Microsoft, Alphabet (Google), Meta (Facebook) and some of the big pharma manufacturers such as Pfizer and MSD.

Of course, it would be misguided to worry about a sudden exodus - none of these firms are going to fold up their tents in the morning and withdraw from Ireland, taking hundreds of millions of annual tax revenue with them.

Indeed, many are doubling down on their presence here. Apple recently announced a new European engineering and test facility for Cork.

Alphabet's CEO told RTÉ News in an exclusive interview a few weeks ago that the company will continue to invest here.

Meta is currently completing a new campus for its employees in Ballsbridge in Dublin.

Ireland also continues to be a global leader in biopharma, the third largest global exporter of pharmaceuticals, with nine of the top ten operating here.

Their love affair with this country is unlikely to suddenly end.

And when you expand beyond the top ten to the top 100, more and more large indigenous and embedded firms like CRH, Ryanair, Kerry Group and Smurfit Kappa, who pay tax here, come into the list of big corporation tax payers.

But all that said, it is worth asking whether it is a healthy situation to be so exposed to and reliant on a very small discrete group of companies, particularly when many are businesses that are controlled outside of the country?

Should we have such a concentration of our corporation tax revenue in a relatively modest collection of massively powerful organisations?

For starters, it gives them disproportionate leverage when it comes to the corridors of power and influence.

Second, corporation tax revenue is notoriously volatile and unpredictable, something the Department of Finance has acknowledged in the past.

But more than that, further big reforms to the global corporation tax order are coming down the tracks.

If those OECD brokered amendments get the go-ahead (which is looking less of a certainty), Ireland's corporation tax rate for large firms will rise to 15% and there will be further alterations to where many big firms pay their tax.

The Department of Finance has estimated that this could cost Ireland €2 billion a year in lost corporation tax receipts.

But it could be more (or perhaps less) and effect some firms more than others. The truth is we don’t really know.

And it isn't just corporation taxes that are heavily dependent on a few big players.

Foreign multinationals are responsible for generating 53% of employment taxes that come from corporate employers too.

Therefore, if those large companies were to scale back in any way, it wouldn't just be the corporation taxes that would fall. Income tax, PRSI and USC would fall too.

Arguably there isn't much we can do to reduce the disproportionate influence of a few giant firms on our corporation tax receipts, other than by trying to broaden the tax base in other non-corporation tax areas and by supporting the growth of indigenous firms.

Nonetheless, although it is without doubt that such enormous businesses are a hugely important part of Ireland's economy in a myriad of ways, Revenue’s report this week does raise some searching questions about growing over dependence on a small few.

And it also raises queries about what might happen if one day Ireland's good fortune was to turn and the corporation tax tide were to go out.