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