Four life-sentence prisoners have been behind bars for over 40 years.
According to Freedom of Information figures released to Newstalk, 16 have been in jail for at least 30 years.
A life-sentence prisoner can apply for parole after 12 years, allowing them to leave jail under temporary release - but many have been in prison for a lot longer than that.
The average time a life-sentence prisoner spends in jail has increased in recent years and in 2020, it stood at 20 years.
There are a total of 362 prisoners currently serving life sentences in Irish jails – 352 men and 10 women.
Some 65 men have been behind bars for at least 20 years.
Eleven have been in jail for over 35 years, with four in prison for over 40 years.
The longest-serving prisoner is the rapist and murderer John Shaw.
The 74-year-old from England has been locked up since 1976 when he was arrested for the abduction, rape, torture and murder of two women in Ireland.