Over the past five years, the Department of Finance underestimated how much it would collect in corporation tax by an average €1.1 billion a year.
The finding is contained in a technical paper published by the Department this afternoon.
The report notes that corporation tax receipts more than doubled since 2014. Last year, €10.4bn was collected, an increase of 125% since 2014 when €5.8bn was collected.
This week, the Minister for Finance Paschal Donohoe told an Oireachtas Committee that early indications were that the final figure for corporation tax year this year would also come in higher than expected.
The report recommends an 'extra degree of caution' in planning tax and spending decisions based on 'elevated' levels of corporation tax.
The report finds some of the unpredictability is down to the fact that 80% of corporate taxes are paid in just four months -May, June, October and November- making it difficult to forecast an exact figure.
Changes in accounting rules over the past five years have also resulted in unpredictable windfall returns.
Corporation tax is also highly concentrated. The biggest 10 companies accounted for 45% of revenue. Also, over 70% of revenue came from just three sectors: manufacturing, finance/insurance and information & communications.
The report also found that multinationals availed of allowances and tax credits for capital investment and research and development. In 2017, €49 billion of the €67 billion in plant and machinery and intangible (things like software patents) capital allowances were attributed to foreign owned multinationals.
In other words, there has been a huge investment by multinationals here for which they received a discount on their tax liabilities. But, it also resulted in certain activities being based here. That means, when those activities make a profit, it adds to our corporate tax take.
Another factor is the number of companies that are no longer making losses and using those losses to offset taxes on their profits.