Fleming, now clad only in a shirt and blanket, refused to do prison work. As a consequence, he was placed in a punishment cell where the blanket was taken from him. In the cold and empty cell, naked except for a shirt, a relapse of his illness soon followed. The prison doctor showed concern and directed that he be provided with adequate bed clothing. This was delivered to his cell.

Fleming decided to intensify the protest and went on hunger strike. Many concessions were offered to him, but the authorities refused to concede the right to wear his own clothes. He continued his protest. It was a display of courage and defiance which other prisoners found inspiring, some of whom responded to his example. The authorities threatened to use the cat-of-nine tails on him and once more left him naked in his cell. This time his health deteriorated to such an extent that the authorities were forced to release him in November 1917.

Upon release, he immediately reinvolved himself in republican activities and consequently, was reimprisoned to serve the outstanding part of his sentence in May 1918. When he arrived back in Portlaoise he was informed that he would not be recognised as a political prisoner. Fleming viewed this as a matter of principle and declared his proposed resistance to all prison rules. He was again placed naked in his cell from 7am to 8pm without bed or bed clothes. Every morning the screws forcibly attempted to dress him in prison uniform, but without much success. Through out these ordeals he was frequently brutalised but regardless of the personal consequences, he continued to resist. In desperation, the prison authorities put Fleming in iron manacles in an attempt to keep the prison uniform on him. They also used a body belt to strap his arms to his body. He was left like this throughout the entire day, making it necessary for him to eat his food from the floor like an animal.

On occasion, Fleming, in a display of unimaginable strength, managed to break out of the iron manacles and the body belt, remove the convict's uniform and tear it to shreds. In retaliation, the authorities placed him what were known as muffs. These were leather flaps and straps which bound the body and arms so rigidly that it paralysed the muscles and tendons. Somehow he managed to break out of these too. A special set of muffs were then placed on him which made it impossible for him to eat. He responded by going on hunger strike. When the Republican Movement was made aware of this, they ordered him off the hunger strike. He was then placed under medical supervision and an attempt was made to have him certified insane. It failed. Fleming continued his protest, breaking loose from strait jackets and ripping up convict's uniforms whenever the screws managed to get them on him. Sometimes it took up to eight screws to continuously watch and restrain him. His constant battles periodically affected his health and on many occasions the doctor's intervention almost certainly saved his life.

In the summer of 1918, Shortt, the Chief Secretary for Ireland visited Portlaoise Prison. The result of his visit was the construction of a special cell which later became known as 'Fleming's Cell' or as it was then known 'Shortt's Stronghold'. This cell was designed to enable the screws to leave Fleming without a special guard. Located on the ground-floor (E1), the cell had the ceiling removed, thereby incorporating the cell directly overhead (E2) into one cell. To deny Fleming access to broken glass, which he frequently used to cut his way out of the straitjackets, the window of the ground floor cell was bricked up. The only natural light to the cell was coming from the window of the cell overhead which was approximately 16 feet from the floor.

A new ceiling was built, sloping at an angle of 45 degrees from the base of the overhead cell door to the ceiling of the overhead cell. It had a light bulb in the centre with a special spy hole to observe Fleming from above. From this position it was like looking into a pit. A special radiator was put into the cell to protect Fleming from the cold which threatened to make his bad health even worse.

This radiator became known as 'Fleming's piano' following Fleming's habit of interfering with it and rendering it unworkable. The screws were continually trying to repair it, but Fleming always found some novel way of disabling it. The authorities resorted to encasing the radiator in a wooden jacket to prevent Fleming from gaining access to it, but this too failed. If there was a way around it, Fleming found it. Finally, in an attempt to frustrate Fleming's efforts, the radiator was encased in cement. With the authorities mistakenly believed their problems were solved.

Fleming spent the first day in this cell tearing bedclothes to shreds, destroying the mattress and bedspring. He even smashed the light bulb with the rubber chamber pot. This rubber pot was intended to prevent Fleming breaking the light bulbs and glass in the cell windows. When the screws opened the door for the first time, however, Fleming tossed all the broken material, torn blankets and uniform out onto the landing. A special concrete bed covered with timber was introduced. He was again placed in a straitjacket and the dreaded muffs. On occasion he even had his hands handcuffed behind his back. Despite all these restrictions he continued to amaze everyone by extricating himself from them. Many a time he had to be shifted to the hospital cell, having become ill due to his exertions. Once recovered and back in the special cell he resumed his one man war against the policy of criminalisation. Finally on 1 January 1919, the British authorities deemed Fleming entitled to political status. He was transferred to Mountjoy Jail from where he successfully escaped on 29 March 1919.

This specially-constructed cell remained as it was then until 1980. It was to serve the same brutal purposes against IRA prisoners in the 1940s and also in the 1970s. Padraic Fleming found himself imprisoned again in 1923 during the Civil War, this time by former comrades, in Mountjoy Prison. He was in the cell next to Ernie O'Malley. O'Malley in his book, The Singing Flame refers to Fleming's bad health:

During the night he often had heart spasms, his body grew rigid, his speech became incoherent. If anyone would touch him during an attack he would regain control; if not he would feel as if his heart was slowing.

He could lie inert for half the following day as a result. To help Fleming during these seizures it was necessary for someone to reach in and touch him physically. A small hole was made in the wall between the two cells for this purpose and O'Malley had on occasion to do just that. Fleming's heart had been seriously damaged by the excessive exertions he had put his body through in defying the Portlaoise Prison regime's efforts to break him. Padraic Fleming's tremendous strength, courage and indomitable spirit remain an example and an inspiration to all political prisoners in their struggle for justice and political recognition.

Fleming had a long career as a republican activist. After the signing of the treaty on 12 December 1921 and prior to the Civil War he acted under Michael Collins. Tim Pat Coogan mentions Fleming's involvement in the execution of Sir Henry Wilson in June 1922 in London in his book on Michael Collins. Apparently Peig Ni Bhraonain who was one of Collins' couriers, met Fleming who gave her a letter purporting to be an offer of a job as a waitress in Woburn House, London. In fact it was instructions for Liam Tobin who met her at Euston Station in London. One week later, Sir Henry Wilson was shot dead.

Again during the Civil War Fleming was active with the republican forces. His involvement continued long after this, resulting in his imprisonment with Ernie O'Malley in Mountjoy Jail.

from The Captive Voice/An Glor Gafa, Winter 1994, p. 11