Guildford Four member Gerry Conlon wrote to the Irish government describing his "living hell" in prison, declassified documents have shown.
A letter written by Mr Conlon 12 years into his life term following the Guildford pub bombings in 1974 suggested he was thinking of killing himself.
The wrongly-convicted men served 15 years before their release in 1989.
It is widely regarded as one of the UK's worst miscarriages of justice.
The letter was written from HMP Long Lartin in 1987 - seven years after the death in prison of Mr Conlon's father Giuseppe, who was wrongly convicted on explosives charges after the IRA blasts.
Mr Conlon wrote to the Irish tanaiste and foreign affairs minister Brian Lenihan: "If nothing is done to help us, I must face another 18 years of a 'living hell'.
"I can assure you that I do not intend to serve it, I would much rather join my dear father.
"I can see that if my plight is not resolved in the near future that I will have to decide which form of protest I must take.
"This is not something I want to do but you can only suffer so much."
After reading the letter, Mr Conlon's sister, Ann McKernan said: "Who could blame Gerry if he contemplated suicide, an innocent man who watched our innocent father die in a British prison?
"Gerry suffered so much with great dignity. I'm glad this letter has come to light because it shows that, even in the deepest despair, Gerry had a firm grip on the issues over his arrest."
Mr Lenihan's reply a month later tried to assure Mr Conlon of the support of the Irish government.
He wrote: "I understand the frustration which you now feel after 12 years of imprisonment. The widespread public sympathy for your predicament and support for your case, however, (may/can/should) be a source of encouragement for you."
The Guildford Four
- 5 October 1974 - IRA bombs explode in two pubs in Guildford, Surrey, killing five people and injuring scores more. Guildford was known as a "garrison town", with several barracks nearby
- 22 October 1975 - Paul Hill, Gerry Conlon, Patrick Armstrong and Carole Richardson - the Guildford Four - jailed for life
- 19 October 1989 - After years of campaigning, the Court of Appeal quashes the convictions, ruling them as unsafe
- 9 February 2005 - Prime Minister Tony Blair formally apologises to the Guildford Four for the miscarriage of justice they suffered
Five people died and more than 65 were injured in the Guildford explosions.
When the Guildford Four were sentenced, trial judge Mr Justice Donaldson told them: "If hanging were still an option, you would have been executed."
The Balcombe Street IRA unit admitted responsibility for the explosions in 1976, although no-one else was ever charged.
After the BBC obtained papers at the National Archives under FOI, Mrs McKernan hired lawyers who said the documents showed "possible alteration or suppression of evidence".