Taoiseach Leo Varadkar has told Egyptian president Abdel Fattah el-Sisi that his priority is to secure the return of Ibrahim Halawa to Ireland as soon as possible.
Mr Halawa will have to wait another three weeks for the verdict in a trial over protests in Cairo in 2013.
The 21-year-old Irishman was expecting to learn his fate at a hearing today, but his legal representatives have said the judgment has been put back until 18 September.
In a 25-minute phone call this afternoon, Mr Vardakar expressed his disappointment to President Sisi at the further adjournment.
The president said his government could not interfere in the judicial process, but gave Mr Vardakar his word that he would resolve the matter swiftly with Egyptian constitutional and legal framework once the trial concludes and a verdict is known.
Mr Halawa, the son of a prominent Muslim cleric in Dublin, Sheikh Hussein Halawa, was imprisoned after being detained in a mosque near Ramses Square in Cairo during protests over the removal of elected president Mohammed Mursi in August 2013.
Mr Halawa's lawyer, Darragh Mackin of KRW Law, a human rights law firm based in Belfast, said the latest delay was "horrific".
Speaking on RTÉ's Drivetime, Mr Mackin said that at this stage the grounds for the adjournment are entirely unclear.
He said the big issue seems to be that of a security concern at the prison where Mr Halawa is being held, which he says is unrelated to Mr Halawa's case.
Hearings in the mass trial involving Mr Halawa and 493 others have been adjourned and delayed numerous times over the past four years. The prosecution case ended last month.
No specific evidence was produced relating to Mr Halawa, KRW Law has said.
Along with scores of others he is accused of murders, bombing, possession of firearms and explosives, arson, violence against police and desecration of Al Fatah Mosque.
Mr Halawa's sister Somaia has said: "It is truly upsetting and heart-breaking to constantly keep having our expectations and hopes shattered into a million pieces.
"It is not just Ibrahim's life that is being played around with but also the entire family's life. We would ask that our family are allowed some time to ourselves during this difficult period."
Minister for Foreign Affairs Simon Coveney expressed disappointment and frustration at the news of the adjournment.
In a statement, he said: "I am frustrated at the news that the final hearing in the court case involving Irish citizen Ibrahim Halawa was today deferred.
"The Irish Ambassador was in court for this morning's proceedings, and our understanding now is that the verdicts in the case will be announced on Monday, 18 September.
"This is very frustrating for Ibrahim Halawa and for his family, who were expecting some definitive news today, and I fully share that sense of disappointment and frustration at yet another delay in this long-running case.
"Nonetheless, we will be continuing to work in support of Ibrahim and his family and doing everything we can to ensure that he is able to return to Ireland as soon as possible once the trial ends."
Fianna Fáil leader Micheál Martin has said it is very worrying that Mr Halawa's trial has continued for so long, describing it as unacceptable.
Speaking on RTÉ's News At One, Mr Martin said he hopes the Taoiseach may be in a position to clarify the situation and that the phone call between Mr Varadkar and the Egyptian president will go ahead today.
He said it is with dismay and disappointment that people are hearing that a verdict in his trial has been postponed for a further three weeks.
"I would hope that the Taoiseach may be in a position to clarify things later today, but that his phone call will go-ahead with President El-Sisi," he said.
"He needs to say that our patience has run out. In my view it is very worrying, that we haven't been in a position to press this case as effectively as one would have thought.
"I don't think anyone would have predicted it would have gone on for so long."