The independent Galway County Councillor, Michael Fahy, who was admitted to University College Hospital in Galway last night is still being treated today.
A relative who visited him said he had complained of chest pains and was having tests carried out.
A hospital spokesperson described Mr Fahy's condition as comfortable. It is expected he will be transferred to a hospital ward later this afternoon.
Councillor Fahy, 56, was due to start a 12-month prison sentence today following his conviction of misappropriation of council funds.
He was handed the prison term and fined €75,000 for theft and fraud offences last month.
The councillor from Caherduff, Ardrahan in Co Galway, was due to present himself at Loughrea Garda Station today, from where he would have been taken to Castlerea Prison.