The average number of prisoners in custody daily rose by eight per cent to 3,911 last year, according to the Irish Prison Service annual report.

Since the beginning of this year, numbers in custody on any given day have been higher than 4,000.

Some 349 prisoners were serving life sentences for homicide while another 237 were serving defined-term sentences of 10 or more years.

The increases have given rise to concerns that the prison system cannot deal with the rising numbers of inmates.

Justice Minister Charlie Flanagan pointed to the fact that committals to prison fell significantly during last year.

About 455 people were committed to prison for failing to pay a fine last year, compared to 2,261 in 2017, he said.

As so many fine defaulters have been removed from the prison system, the total number of committals to jail declined from 9,287 in 2017 to 8,071 last year, a fall over about 13 per cent.

“We must continue to ensure that violent offenders and other serious offenders are committed to prison, while at the same time switching away from prison sentences and towards less costly non-custodial options for non-violent and less serious offenders,” he said.

Before the Fines Act was introduced, people who were jailed for failing to pay a fine were included in the prison service computer system at a jail and then released, producing phantom prisoners that only existed in official records.

While their absence has reduced official records for the number of prison committals, it has not affected the daily prison population.