Ibrahim Halawa, the son of the most senior Muslim cleric in the Republic of Ireland, was arrested during a siege on the Al-Fath mosque in Cairo in 2013.
The mass trial of Mr Halawa and more than 400 others began in March after being postponed five times since his arrest.
Mr Halawa, 19, could face the death penalty if he is convicted.
Mr Halawa was on a family holiday to his parents' homeland when he and three of his sisters were arrested by Egyptian security forces during a crackdown on protests in the country's capital.
He was 17 at the time.
His family said he had taken refuge in the building during violent clashes between supporters of ousted president Mohammed Morsi and the security forces.
His sisters were allowed to return to Dublin in November 2013.
On Sunday, a court in Cairo postponed the mass trial of more than 400 defendants, including Mr Halawa, until 4 October.
Mr Halawa's trial has now been postponed eight times.
RTÃ‰ reports that Irish Minister for Foreign Affairs Charlie Flanagan has expressed his disappointment that Mr Halawa's case has been further postponed because of the non-attendance of a number of witnesses.
He said he had regularly emphasised to his Egyptian counterpart Same Shoury the importance of due process in the case and the Irish government's concern at the continued detention of an Irish citizen while awaiting trial.
In Cairo, to attend the trial, were Mr Halawa's sister Khadija, his solicitor Darragh Mackin and MEP Lynn Boylan, but they were not inside the courtroom.
Mr Mackin said he and colleague Gavin Booth, who are both from the Belfast-based law firm firm KRW Law, were told they were not allowed access to the court.
"Not allowing a lawyer to access his client's hearing is gravely concerning and belies any suggestion that fair trial requirements are being met," he said.
"Our client, Ibrahim Halawa, has now been held for almost two years without trial.
"We are concerned that there has been yet another delay, this time of a further two months. Ibrahim was aged 17 when arrested - he is now 19. "
The trial of a Dublin teenager held in an Egyptian prison for almost two years has been adjourned until 4 October.