Jailed Irishman Ibrahim Halawa has warned he has been behind bars for so long that he is scared of freedom.
The 21-year-old - incarcerated in an Egyptian prison for four years over Muslim Brotherhood protests in Cairo in 2013 - is said by his sisters to have become withdrawn and is losing sense of things.
Somaia Halawa, one of the sisters who has spearheaded the campaign for her brother's release, said it is time to see a result from Government pledges to work for his release.
"Four years is too long for someone who is innocent," she said.
"Four years is too much for our brother, the 17-year-old boy who has become a man locked away from the world, his family, friends. Four years of which Ibrahim has given of his body and his spirit to stay alive. When will Ibrahim be home?"
At a protest outside the Egyptian embassy in Dublin to mark the fourth anniversary of his detention, she welcomed a solidarity message from Foreign Affairs Minister Simon Coveney.
But she said: "I think it's time to see a result.
"If Ibrahim is able to stay strong today I don't think he will tomorrow.
"There's a time that Ibrahim will lose all hope, especially now that he is waiting day by day for the Government to fulfil its promise to help him."
Hearings in the mass trial involving Mr Halawa and almost 500 others have been adjourned and delayed numerous times over the past four years. The prosecution case ended last month.
The next hearing is expected on Sunday, with a judge expected to deliver a verdict on the defendants in weeks.
Egyptian president Abdel Fattah el-Sisi has said Mr Halawa could be released, but only once the trial has run its course.
Another sister, Omaima, revealed details from a letter written by her brother in recent days.
"I'm scared of the light of freedom," he wrote. "Where my life was one day stopped for four years - it will probably return to continue except everything will have changed."
Mr Halawa added: "I was imprisoned as a kid and will be released as a man.
"Two paths - one that was not finished and one that I started alone.
"Yet I will be expected to be released from the suffering of prison to the vagueness of life."
Omaima Halawa said she believes her brother has become so institutionalised that he is scared about being freed.
Dozens of protesters, including members of Amnesty International Ireland, the Union of Students of Ireland, United Against Racism and campaign group Uplift, joined the Halawa sisters and their children at the embassy.
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 as the Muslim Brotherhood protested over the removal of elected president Mohamed Morsi in August 2013.
He has been tortured and gone on hunger strikes during his imprisonment.