The drug is used to treat heroin addiction
Over half of female prisoners are on methadone, which is normally used to treat an addiction to heroin.
According to freedom of information figures, 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.
Some 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.
83 of the 145 women in the Dochas prison in Dublin at the end of last year were on methadone - and 21 of the 44 in Limerick women's jail.
Former governor of Mountjoy Prison, John Lonergan, is not surprised:

Further Increase Expected In Number Of Landlords Leaving Property Market, As Price Of Three-Bed Semi Rises In Kildare
Over 1,700 Garda Checkpoints In Operation Over Christmas Period, With Naas RPU Detecting Multiple Speeding Motorists
€1.2m Approved For Zipline And High Ropes Adventure Centre At Donadea
Christmas Appeal To Adopt Dogs As Euthanasia Figures Rise
Speed Enforcement To Increase In Kildare With 21 New Camera Zones To Be Rolled Out In January
Over 400 Motorists Caught Speeding So Far Today, Including A Driver In Kilcock
Transport Minister In Discussions With Irish Rail About Extending Late-Night Trains Beyond Christmas
Tadhg And Mary Should Be Looking Forward To Christmas, Say Gardai As Fresh Appeal Launched