The Real IRA leader Michael McKevitt has begun an appeal against his conviction in the Court of Criminal Appeal.

The issue of the credibility of David Rupert, the State's main witness at the trial Mr McKevitt, will be of huge significance during the four-day appeal which opened this morning.

McKevitt, from Beech Park, Blackrock, Co Louth was jailed for 20 years by the Special Criminal Court in 2003 for directing the activities of a terrorist organisation. He was also convicted of membership of the Real IRA.

Michael McKevitt has raised 42 grounds of appeal over the conviction.

He was the first person in the country to be convicted under legislation which was introduced in the wake of the Omagh bombing.

At his sentencing in 2003 the Special Criminal Court pointed out that the offences for which he was convicted were outside the date of the Omagh bombing. 

The court said it must not be seen to seek revenge for that atrocity and would not seek to do so.

The main prosecution witness at the trial was David Rupert, an FBI and MI5 agent who testified against McKevitt.

Lawyers for McKevitt will raise issues in relation to disclosure of information during the trial, the way the trial court dealt with question of privilege and the failure, it is claimed, of that court to address issues of the credibility of Mr Rupert.

There is heavy security at the Four Courts for Michael McKevitt's appeal.