Revenue has already begun identifying homeowners who have failed to sign up for the controversial local property tax (LPT), the agency has confirmed.
As the extended deadline for voluntary registration loomed, chair of the Revenue Commissioners Josephine Feehily warned that "non-engagers" will be contacted within the next 10 days.
She confirmed that 1.52 million forms had been returned by Wednesday afternoon - ahead of the 8pm deadline - and predicted the overall compliance rate would exceed 80%.
"The next steps in this project include activating the various payment options chosen, including by sending files to employers and the relevant Government departments to begin LPT deductions at source," Ms Feehily said.
"We are now moving quickly into compliance mode and have already begun to identify the non-engagers for follow-up action. We owe no less to all the compliant taxpayers who have voluntarily filed their returns."
Homeowners who miss the deadline to sign up for the self-assessed tax, which will come into force on July 1, could have the money "deducted from source" as early as August. They could see Revenue take the money owed directly from salaries, pensions and benefits.
"First of all, we have decided that we will begin by identifying the non-engagers who are in the PAYE sector. We will send them a reminder letter," Ms Feehily said. "The answer to how we will proceed is very carefully and in stages."
She said they will be given "a very small window" to file online. Failing that, Revenue will contact their employers to arrange deduction from their salary, which would be spread over a number of months.
Ms Feehily said all non-engagers will be sent reminder letters in the first instance, which they will receive in early June. She added that no one will have a penalty imposed automatically without engagement from Revenue, which will include reminder letters and a warning.
Self-employed homeowners who fail to register will be blocked from receiving a tax compliance certificate until they comply and pay. Ms Feehily added: "This will come after appropriate warnings and appropriate engagement. That comes further down the road. We have a backstop for self-employed people. If they haven't paid the LPT by the income tax return, they will have a surcharge on their income tax return."