New Exchequer figures show tax revenues were more than 2% below target last month.
The State took in €98m less than expected during July.
The Department of Finance said the figures were mainly due to an under-performance in VAT receipts, which were 3.3% lower than expected.
Corporation taxes and excise duties also came in lower than expected.
Tax partner at Grant Thornton Peter Vale said: "If this trend accelerates post Brexit, we could see the VAT figures falling further behind at the end of September.
"A resultant drop in VAT receipts could impact on the scope for tax cuts or spending increases in the Budget."
However, the tax take in the first seven months of €26.6bn is still 2.5% (€644m) ahead of expectations, and 8.5% higher than this time last year.