A nightclub owner, a doctor and an insurance company are among the tax defaulters hit with bills of nearly €10m in the last few months.
In its latest tax defaulters list, Revenue has said 43 settlements were made in April, May and June while more than €6.5m remained unpaid as of June 30.
Twenty-one of the settlements were for amounts exceeding €100,000, of which six were over €500,000.
Fines and penalties of more than a million euro were also imposed by the courts on people under-declaring tax or not declaring it at all.
There were 185 of those cases.
Revenue said: "Revenue vigorously pursues collection/enforcement of unpaid settlements.
"In some cases, collection/recovery of the full unpaid amount will not be possible (for example, company liquidation)".
A website consultant from Dublin has the highest bill on €888,000.
Michael P Doyle from Dartmouth Road in Ranelagh owes the under-declared income tax, but none of it has been paid yet.
Next on the list is medical service providers Mediserve Homecare Limited from Fitzwilliam Place in Dublin which owes €858,000 in underdeclared tax.
A vehicle test centre operator Sean O'Leary from The Faythe in Wexford was told to pay nearly €706,000 for non-declaration of income tax and under-declaration of VAT, while a Cavan-based wholesaler is to pay over €852,000 for the same offence.
A car dealer Noel Turley from Mountain North in Athenry, Co. Galway is to pay €799,000 for under-declaration of VAT.
There were also nine such cases where a court determined the penalty - with the total amount reaching just over €591,000.
And there were 176 where a court imposed a fine, totalling over €550,000.
In one instance, a Cork-based subcontractor faced seven charges for failure to lodge income tax returns.
He had to pay a fine of €28,000.
While a Wexford farmer was fined €17,500 for her failure to lodge VAT returns.
A window installer in Wexford paid a penalty of nearly €125,000 for under-declaration of income tax and non-declaration of VAT.
Also, a Monaghan-based property developer was ordered to pay €125,043 for non-declaration of Capital Gains Tax.