Ireland's president, Michael D Higgins, is under growing pressure to release from prison a friend and cancer-stricken peace activist whose cause is being championed by some of the country's most prominent artists.

Family and supporters of the ailing 79-year-old playwright Margaretta D'Arcy have called on Higgins to pardon the anti-war campaigner and secure her early release from Limerick jail, after she was given a three-month sentence for disrupting US military flights at Shannon airport in Ireland's south-west.

D'Arcy, who also has Parkinson's disease, was jailed earlier this month after she refused to sign a bond guaranteeing that she would no longer try to disrupt the flights. The US Air Force uses Shannon as a stopover when transporting troops back from Afghanistan and other American bases in the Gulf and the Middle East.

The writer's continued incarceration poses a major personal crisis for Higgins and his wife Sabina. Last Sunday Sabina Higgins came under fire in sections of the Irish media after she visited D'Arcy in jail. Critics accused Ireland's first lady of breaking the Republic's age-old presidential code – whereby the nation's figurehead and their family do not stray into party politics once elected to office.

But D'Arcy's son, Finn Arden, said Sabina Higgins's visit had given his mother a morale boost. "It would be great if she were pardoned and released as soon as possible," he said. "I spoke to her on Thursday and she was very fragile, pale and wobbly. She is meant to be having three-month check-ups and just before Christmas more tumours were found on her bladder.

"A pardon would be very welcome, but there is also the question of the bond. If that was changed to allow her to protest against the military presence in Shannon it would be better."

The bond she refused to sign late last year barred her from entering "unauthorised areas" of the airport, including its runway, where she has staged two sit-down protests.

Dylan Tighe, a Dublin musician who has co-ordinated protests against D'Arcy's imprisonment, said it was "perverse that the bankers and politicians who ruined this country are free while an elderly peace activist is jailed".

Up to 500 writers, artists, musicians, actors and dramatist have signed a petition in Ireland urging justice minister Alan Shatter to release D'Arcy on humanitarian grounds. The group, which includes the award-winning novelist Belinda McKeown and the playwright Peter Sheridan, have also criticised Ireland's Arts Council for refusing to intervene in the case.

Tighe said: "The president could use his powers to ask the government to pardon and free Margaretta. We would support that move 100%. It would be a great boost for the campaign if the president went to the government to get a pardon for her."

The Irish presidency refused to comment on any potential use of a pardon to grant D'Arcy an early release. A spokesperson for the president said: "Sabina Higgins's visit was in a personal and private capacity to a person she has known over many years."

Although Michael D Higgins has the power to suggest a pardon for a prisoner, it has to be ratified by the Irish cabinet.

Not everyone in the region around Shannon airport is opposed to the US military presence. Pat McMahon, who has served on Clare county council for 37 years, told the Observer that "the silent majority in Clare, Limerick and around Shannon airport are pro-American".

The Fianna Fáil councillor said: "The people who come down to protest against the American military aircraft don't come by and large from this area of the country. The majority who live down here have been pro-American for decades because of traditional links to the US and the presence of so many US companies that create thousands of jobs around us. I have been a public representative for 37 years and I can tell you that I don't detect any anti-American feeling around Shannon."

Arden, meanwhile, stressed that his mother was in good spirits despite her physical weakness. "She told me she is concerned about the conditions for the other women in Limerick jail," he said. "When she gets out I have no doubt this will be her next campaign."