There are 71 life sentenced inmates in the Midlands Prison.
50 prisoners who were given life sentences have been in jail for at least 20 years.
New figures also reveal 11 of them have been behind bars for at least 35 years.
The average amount of time a life-sentenced prisoner spends in custody in Ireland is about 18 years, according to the Parole Board latest figures.
But new figures, released under the Freedom of Ireland Act by the Irish Prison Service, show many inmates have been in jail for much longer.
There are currently 355 life-sentenced prisoners in 13 jails across the country - 345 men and 10 women.
17 have been behind bars for at least 30 years - with two of them in custody for over 40 years.
Former governor of Portlaoise and Mountjoy Prison John Lonergan says there are two reasons people would be in jail for so long:
Of the 355 life-sentence prisoners in jail at the moment, 71 are in Midlands Prison in Portlaoise and 65 are in Mountjoy Prison.
Midlands Hospital Experiencing High Admission Numbers
Founding President Of TUS To Step Down Next Year
Westmeath Father Loses Job After Using HSE Funds For Home Upgrades
Higher Education Minister Visits Offaly
Midlands Woman Pays Tribute To Sister After Manslaughter Sentence
FTD Brothers Continue 32 County Marathon Challenge Through The Midlands
Fresh Restoration Works Begin At Westmeath Castle
Nearly 1,000 Homes And Businesses Hit Affected By Power Outage In Midlands Town
TUI Says State Should Pay For School Devices
Musicians Gather In Offaly Celebrating Fr Liam Lawtons Music
Minister For Health Wants Closer Working Relationship For Midlands Hospitals
US-Iran Peace Deal Nears As Talks Continue
Westmeath Activist Calls Out Government After Being Detained By Israel
Laois TD Voices Concern Over Appeal For Proposed Windfarm
HSE Still Formally Settling Offaly Hospice Dispute
Weeklong Heatwave Hits Midlands
Uisce Eireann Agree To Examine Pipeline Solution For Westmeath Town
Offaly's Comhaltas President Meets King Charles In Belfast
Laois TD Demands Deer Be Tested For TB
Health Minister Opens Newly Extended ED Department In Laois