The State's fiscal watchdog has estimated that almost half of the Corporation Tax collected by the State is paid by three multinationals.
In 2024, these large companies paid around €13 billion or 46% of the tax, according to the Irish Fiscal Advisory Council.
While the organisation has not named the three multinationals, they are understood to be Apple, Microsoft and pharmaceutical group Eli Lilly.
IFAC said that Corporation Tax almost doubled between 2021 and 2024, even when back taxes paid by Apple were excluded.
The council said this sharp rise was largely driven by the increased payments from the top three players.
It is estimated that the two technology companies, Apple and Microsoft, paid the equivalent of almost 40% of total Corporation Tax receipts.
IFAC economist Brian Cronin said the "research highlights how reliant Ireland’s Corporation Tax has become on just three companies".
"These companies continue to perform strongly, but their profits and the taxes they pay remain subject to significant uncertainty.
"As a result, corporation tax receipts could be substantially higher or lower than current levels in the medium term," he added.
It is expected that profits from Apple and Microsoft will increase due to advances in artificial intelligence and growing demand for their products and services.
In the pharmaceutical sector, Eli Lilly is likely to continue to benefit from a surge in demand for weight-loss and diabetes medicines.
Watch: 'Sustainable' services needed from Govt, not just when taxes ‘booming’ - IFAC
IFAC Chair Seamus Coffey said the Government should be able to provide services on a sustainable basis, not just when "booming tax revenues" are coming in.
He told RTÉ's Morning Ireland that relying on a small number of companies is a risk.
"You're looking at providing Government services on a sustainable basis, not just when this money is coming in.
"There are clear priorities that the Government has identified that it wants to target, and there's no question that it can do that, but it should be able to do so on the basis of sustainable revenue, tax revenues that are there - regardless of the performance of two or three companies, or boardroom decisions of two or three companies, or a social media post from the President of the United States.
"So when it comes to capital spending on housing, when it comes to investment in health, education etc, these are things governments should be doing all the time, not just when these booming tax revenues are coming in," said Mr Coffey, who is a economics lecturer at University College Cork.
