Over one-third of female prisoners are on methadone, which is normally used to treat heroin addiction.
In total over one in every eight prisoners in Irish jails is on methadone.
On October 9th, there were 3,741 prisoners in 13 jails across the country.
According to Freedom of Information figures, 511 were being prescribed methadone - making up nearly 14% of the prison population.
The substance is normally used to try to wean people off heroin and other illegal opiates.
Women account for the highest amount of prisoners on methadone.
Some 42 of the 131 people in the Dóchas Centre in Dublin on October 9th were on methadone, which is 32%.
The rate was 41% in Limerick women's prison, where 12 of the 29 women in custody were being prescribed the drug.
John Lonergan, a former governor of Mountjoy Prison, says he is not surprised.
"A huge number of women who get into difficulty with the law have many personal issues, including addiction, ever before they get involved in criminality.
"Vast, vast numbers come from the big urban areas like Dublin, Cork, Limerick - where heroin would be widely used - especially amongst the population that would be socially disadvantaged and homeless people".
Of the men's jails the highest amount of prisoners on methadone was in Cloverhill, at 24%, with 22% in Mountjoy.