The High Court has ruled that musicians Foster and Allen must pay around €3m each to the Revenue Commissioners.
Michael Foster had challenged a judgment granted against him in 2008 for almost €3m in tax and penalties arising out of a €1m tax demand.
Tony Allen challenged the Revenue Commissioners action for summary judgment against him over a demand for income tax and penalties totalling almost €3.5m.
The claim for income tax goes back to the 1980s.
Mr Justice John Hedigan rejected their challenge.
He described Mr Foster's action as a "hopeless application" and said he had delayed too long in bringing it.
He found that Mr Allen had not established that he had a real defence that was likely to succeed to the application for judgment.
