Three judges are due to begin hearing an appeal this morning against Thomas 'Slab' Murphy's conviction and the 18 month sentence handed down.
The prominent Republican, of Ballybinaby, Hackballscross, Co Louth, was prosecuted for not filing tax returns between 1996 and 2004.
Nine charges of tax evasion were brought against Mr Murphy following a raid of his farm in Louth in 2006.
Bags of cash were found in an outhouse on his land -which straddles the border with Northern Ireland.
He was tried before the non-jury Special Criminal Court amid fears it would not be possible to effectively administer justice in the ordinary courts.
The 66-year-old claimed he was the victim of an identity theft and denied having anything to do with the farm, saying that his brother Patrick ran it.
He was convicted on all counts and jailed for 18 months.
He is now almost halfway through that sentence, but is due to appeal both the conviction and the severity of the punishment handed down.
With over 50 grounds of appeal, the hearing is expected to take up to three days.