Over half of female prisoners are on methadone, which is normally used to treat an addiction to heroin.
According to Freedom of Information figures released to Newstalk, that compares to around one-in-eight male inmates.
At the end of last year, there were just over 4,200 inmates in prisons throughout the country.
626 were on a methadone treatment programme.
The drug can help you reduce your cravings if you are addicted to heroin.
But a disproportionate number of female inmates were on methadone.
There were 104 on the drug out of 189 female prisoners in jail at the time.
Former governor of Mountjoy Prison, John Lonergan, has said is not surprised.
"It's very consistent with my experience of women in prison, the composition of the women's population," he said.
"[A] very big number of them would have chaotic lifestyles; many of them would have major personality, social [and] family issues.
"Very low self-esteem [and] much at risk and very much on the edge - and all the characteristics and the indicators of heroin addiction," he added.
Some 83 of the 145 women in the Dóchas Centre in Dublin at the end of last year were on methadone, and 21 of the 44 in Limerick Women's Jail.