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 20-year-old could face the death penalty if he is convicted.
The mass trial of Mr Halawa and more than 400 others began in March 2015, after being postponed five times since his arrest.
Mr Halawa was on a holiday to his parents' homeland with three of his sisters when they were arrested by Egyptian security forces in Cairo.
Mr Halawa's sister were allowed to return to Ireland.
The Republic's outgoing Minister for Foreign Affairs, Charlie Flanagan, said he was deeply concerned over the latest delay.
"The constant adjournments in the case are, understandably, a source of concern and frustration for Mr Halawa and his family, and I share their deep disappointment," he said.
"I want to reassure Ibrahim's family of my own and the government's continued commitment to achieving our two objectives: to secure his return to Ireland as soon as possible and to ensure his welfare during his detention."
Gavin Booth, from Kevin R Winters solicitors in Belfast, represents Mr Halawa's family.
He said Mr Halawa remains on a form of hunger strike, taking water and some fruit.
"The European Parliament has taken a stronger stance on Ibrahim Halawa's case than the outgoing Irish government.
"Ibrahim's health is quickly deteriorating on hunger strike. This matter has been ongoing for two and a half years now.
"We need an urgent and unified approach from the Irish government," he said.
Mr Halawa's trial is now due to begin on June 26.
The trial of a Dublin teenager held in an Egyptian prison for more than two years has been adjourned until June.