Prison bosses say this is a direct consequence of changes in the law creating a series of non-custodial sanctions for people who do not pay court-ordered fines.
Provisional figures from the Irish Prison Service (IPS) show there were 9,332 committals in 2017, compared to 15,099 in 2016 — a fall of 38%.
The figures are down 46% compared to 2015, which recorded 17,206 committals.
A person can be committed to prison more than once in a year, so the number of people jailed dropped from 14,182 in 2015, to 12,579 in 2016 and to 7,810 in 2017 — reflecting the same percentage reductions as overall committals.
Responding to queries, the Irish Prison Service confirmed that the decline was due to the changes in sanctions for non-payment of court fines.
“The Irish Prison Service saw a reduction in 2016 in the number of people being committed on fines and there was a dramatic reduction in 2017,” said a spokesman.
“At stages during 2017 we had situations where he had basically no one committed for non-payment of fines.
“The reduction in committals in 2017 is completely attributable to a significant reduction in people committed to custody for non-payment of fines, up to a 75% decrease in this cohort in 2017 compared to 2016.”
The practice of jailing people who failed to pay fines — imposed by courts as a sanction for a criminal conviction — created an administrative and resource headache for the prison service.
The system came into disrepute as the vast majority of fine defaulters were brought to prisons and processed — taking up staffing resources and time — only to be released within hours or the following day because of space shortages.
For this reason, they do not impact on the number in custody, which stood at 3,646 on December 31, 2017, compared to 3,595 on the same day in 2016 and 3,647 in 2015.
Penal reformers long criticised jailing fine defaulters, saying they should not be sent to full prisons for relatively minor offences and that non-custodial measures should be used.
Numbers rose sharply over the last decade — from 2,500 in 2008, to 7,500 in 2011 and to 9,000 in 2014. It peaked at almost 9,900 in 2015, accounting for 60% of all committals in that year.
Committals fell to 8,400 in 2016 after the introduction of various non-custodial options. The main element of this was a new method to pay fines by instalment.
This was provided for in the Fines (Payment and Recovery) Act 2014, but did not become operational until January 2016.
Deirdre Malone, executive director of the Irish Penal Reform Trust, said: “These numbers demonstrate that the commencement of the Fines [Payment and Recovery] Act 2014 is having a positive effect in reducing prison committals.
“Imprisonment for fines default represented a waste of Garda, Courts and Irish Prison Service time and resources and created an illogical and additional burden on an already strained prison system.
“Ending the practice has significantly reduced unnecessary and damaging committals to prison, as well as saving the taxpayer money.”
Ms Malone said they were concerned over IPS figures which showed that the number of people in custody at the end of December 2017 was higher than on the same day in 2016, while the number on temporary release was half that of two years ago.