Ibrahim Halawa was 17 when he was arrested during a siege at the Al-Fath mosque in Cairo in 2013.
He has been accused, along with more than 400 others, of inciting violence, riot and sabotage.
He wrote that his incarceration had taught him to find humanity in everyone and disregard revenge.
Mr Halawa wrote to his family and said 22 September should have been the day his parents had lived their lives to see.
He had intended to study a degree in engineering.
Instead he said he was refused proper medical treatment for severe chest pains and had his head forcibly shaved.
"Today while every graduate throws their graduation hat in the sky to come down with the long waiting dream, I don't see the sky," he wrote. "Because I'm enrolled in a different college.
"A college I did not know I had applied for when I chose to fight for freedom. A college that kidnapped me from life to teach me the principles of real life. A college full of lessons. A lot of which I have learned in dark mornings and nights."
The letter, entitled Graduation Speech, revealed he shares a dormitory cell with 30 others and has a sleeping space just over one foot wide.
Mr Halawa told his family he had learned a deep sense of compassion for others during his time in prison and an appreciation of the simple things in life.
"In this college I'm obliged to live with a broad diversity of inmates. From presidential consultants and college professors to illiterate criminals which taught me to seek the real human being behind every social rank," he said.
"I have learned that absolutism is an invalid way to judge humans, humanity is all about relativism.
"It is a college where the hardest subject is 'finding the forgiveness', as I must stay as a freedom fighter and never became a revenge hunter.
"Even though after all these years my oppressor is yet not convinced to let me graduate from his prison and go home. While for others it's graduation night out, for me it's graduation lights out."
The Halawa family and their lawyers KRW Law have attempted to secure a presidential decree for Mr Halawa's freedom.
His case is due back in court on 2 October.
Ibrahim's sister Somaia said: "Ibrahim's letter leaves no room for more to be said, however it has been 35 days since the presidential decree of law 140 was submitted for Ibrahim, after three years of campaigning for Ibrahim's freedom this is our biggest hope.
"It is time for the Irish government to increase the level of pressure so that decree is as effective as it should be."

Manager Matt Beard told BBC Sport that Williams and Nigeria striker Asisat Oshoala would miss "a couple of months" with hamstring and knee problems.
Defender Satara Murray (also hamstring) is expected to be out of action for about six weeks.
"It's a big blow," said Beard.
"It's the spine of your team - your central midfielder, your centre-back and your centre-forward - but it's one of those things."
The players were all hurt during Saturday's WSL 1 defeat by Notts County and did not feature in Thursday's 2-0 Continental Tyres Cup victory against Everton.
Williams played in all seven of England's matches at the recent World Cup, but Beard said her injury was not caused by returning to league action too soon following the tournament in Canada.
"It was an impact injury," he said. "Unfortunately as she's tried to win the ball, her leg's completely hyperextended, which has caused the injury."
Liverpool are the reigning Women's Super League One champions but are currently only sixth in the table after a disappointing start to the season.
With the league concluding in October, Beard admitted that the new injuries would make defending their title even more difficult.
"They're important players to us but it's going to give maybe one or two of the younger players an opportunity," he continued.
"We'll do as much as we can, while we're still involved in everything mathematically, to finish as high as we can."
Beard also told BBC Sport that he did not think he would be in a position to make emergency loan signings to cover for the injured players.
An Irishman jailed in Egypt has written a letter to his family to mark the date he should have graduated from college in Dublin.
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Three Liverpool Ladies players, including England midfielder Fara Williams, are set to miss the majority of the club's remaining games this season after picking up injuries.