Ibrahim Halawa, who has been in prison in Egypt for four years without trial, has written an impassioned plea to Taoiseach Enda Kenny for his release after seeing another inmate being freed.

Mr Halawa, 21, was detained during Muslim Brotherhood protests in Cairo in 2013.

A mass trial involving him and hundreds of other alleged conspirators has been adjourned more than 20 times.

Mr Halawa said he feels abandoned after Egyptian-US citizen Aya Higazy was released from jail.

"I am currently writing this with the most pain in my life but a worst pain has just attacked me, a mental pain... How much I feel abandoned," he wrote.

"Tell me without hiding behind your finger, how do I feel towards my own government? The one I have hanged the flag of above me in my cell over my 35cm sleeping space.

"I hanged it even though they have left me to die in one of the worst prisons in Egypt without trying to move me. They have left me as a kid beaten and tortured to sit and watch all the other nationalities leave to their home."

Vigil held for Ibrahim Halawa outside Leinster House

Boylan 'very fearful' for Ibrahim Halawa's health

Mr Halawa, the son of prominent Muslim cleric 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 Mohammed Mursi in August 2013.

He cited Ms Higazy's case and claimed her release happened after US President Donald Trump met Egyptian president Abdel Fattah el-Sisi in recent weeks.

She was a co-founder of the Belady Foundation for Street Children and was acquitted of a number of charges including inciting children to join protests.

"After nearly four years of imprisonment I still wonder what trust does the Irish Government have in the Egyptian judiciary system, proven people innocent after taking away three or more years of their lives," Mr Halawa wrote.

"I wonder when will Enda Kenny ever get on to a plane to save the Irish citizen? When will Enda Kenny make Irish citizens feel safe to fly abroad?

"Enda send your child to school and wait four years for them to return and see if they're going to call you dad again."

The letter, dated Sunday, also mentions the names of others arrested on the so-called Day of Rage who have since been released, including Peter Greste, the Australian journalist who was working for Al Jazeera covering the protests when he was detained.

Mr Halawa’s trial is due before the courts again tomorrow.

The Taoiseach wrote a letter to Mr el-Sisi a month ago pleading for Mr Halawa's release on humanitarian grounds.