They claim the inquiry no longer has jurisdiction following the recent Supreme Court decision in the Fitzwilton case.
Mr Ahern is due to appear before the tribunal at the end of the month to be questioned about lodgements in the mid-1990s, following claims that he received £80,000 from developer Owen O'Callaghan in relation to the Quarryvale development.
Conor Maguire SC for the Taoiseach said this investigation is based on the hearsay allegation of Tom Gilmartin whose credibility he says is withering on the vine.
He said Mr Ahern's appearance should be deferred until after the tribunal is compelled to see the fallacy of Mr Gilmartin's evidence.
He also argues that the Taoiseach's finances are outside the tribunal's terms of reference as ruled by the Supreme Court in the Fitzwilton case.
The tribunal will give its decision on this submission on Monday.
