The Revenue Commissioners say customers have to register by 8pm tonight if they want to change how they pay their property tax.
Customers who already pay the tax through salary deductions, or by direct debit, don't have to make contact.
But those who want to pay it in one go, or make cash payments throughout the year, must register by tonight. Approximately one million people paid the tax by lump sum last year.
The Revenue's telephone helpline is so busy it can't take any further calls at the moment.
But Revenue's Vivienne Dempsey said if you're making an effort to comply, you won't be punished for missing the deadline.
"We're taking the view that if you're actively engaging with us, and once you receive those numbers that you act promptly - then, in good faith, we'll take it that you're making every reasonable effort to comply," she said.
