Phelan's family, who said that "his life is in danger," have expressed concerns over his health.
What are the accusations against Phelan?
He was indicted for taking photographs of a burned mosque and police officers and sending them to a British newspaper.
He has also been accused of stealing 900-year-old pottery pieces from a village.
Phelan and his family have denied the charges.
At the first hearing held on February 20, where a state-appointed lawyer represented him, he was convicted to three and a half years in prison.
The duration of the sentence accounted for his health issues and age.
But on February 26, a second hearing was held where the sentence was increased to six and a half years.
'Worsening health conditions'
Phelan's health has "deteriorated considerably" during imprisonment according to his family.
He requires regular supplements for his heart condition, and his family is concerned that his supplies will soon finish.
Phelan's sister said that his eyesight is deteriorating.
Due to his heart condition, France has requested Iranian authorities to provide urgent medical care for Phelan.
An official from Ireland's Department of Foreign Affairs also expressed the government's concern over the case due to Phelan's physical condition.
One of the many 'hostages'
Over the past few years, Iran has arrested dozens of dual nationals and foreigners on suspicions of involvement in security and espionage-related activities.
Currently, six French citizens are detained in prisons by the Iranian government according to the French Foreign Ministry, who the ministry says are "hostages."
Tehran has also been accused by human rights organizations of using such arrests as a means to gain diplomatic leverage from other countries.
These claims have been denied by Iran, which does not recognize dual nationality.
aa/ar (AFP, Reuters)