Mr Phelan is being held in Vakilabad Prison, Mashhad, where conditions are reported to be cramped, with 16 people sharing a cell amid freezing temperatures at night.
It is understood that some of those in Mr Phelan’s cell block were killed during his stay.
On 30 March the Iranian Embassy in Ireland said that reports of prison conditions were baseless, politically motivated and aimed at tarnishing Iran’s image.
They said Mr Phelan was “treated well in a prison, with standard conditions and medical services”.
“Family and media reports about the conditions in his prison cell and his critical health condition have been repeatedly proven to be false and inaccurate,” they said.
“Despite the false reports and inaccurate statements, the major health problem with Mr Phelan is not heart or bone related. However, his illness has not been revealed to the Iranian authorities” for purpose of protecting his personal data,” they added.