The average time in prison served by those sentenced to life reached 20 years last year for only the third time in the history of the State.

News that lifers will spend an average of two decades behind bars emerges after a prolonged period in which life sentences increased but then appeared to peak and fall again.

The average term served by “lifers” released from their sentences in the 1970s and 1980s was only 7½ years. However, since then it has increased markedly.

Research conducted by NUI Galway law lecturer Dr Diarmuid Griffin reveals that by 2001 the average term being served by lifers had double through the 1990s to 15 years.

The average remained at that level or increased in some years to the end of the decade and reached 20 years by 2011, increasing further to 22 years for those released in 2012.

But in 2013 it dropped significantly, to 17½ years, returning to levels that had not been seen for five years.

It was initially unclear if that decrease was a one-off and that overall life terms would continue to rise, or if the life tariff had peaked and begun to fall.

However, further figures for last year supplied to Dr Griffin reveal that the average number of years served by those life-sentence prisoners who were released in 2014 was 20 years.

And while data for a number of years will be needed to establish if 20 years will become the baseline for average sentences, it appears life sentences will remain at historically high levels.

Life-sentence prisoners are jailed until the minister for justice of the day signs off on their release, usually on the recommendation of the Parole Board.

Violent killers who still pose a risk have served sentences of 30 to 40 years and in a small number of cases even longer.

Violent crime

The rise from 139 in 2001 to 319 by the end of 2013 represented a 130 per cent increase at a time when the general prison population rose by only 34 per cent.

The rise in the number of life sentences reflects the increasingly violent nature of crime, especially among organised criminals, as well as the success of the Garda in bringing criminals before the courts.

Dr Griffin’s study found that of the 315 life-sentence prisoners in July 2013, 297 were serving the mandatory life term for murder.

The remaining 18 were serving discretionary life sentences imposed for unspecified offences which could include sexual offences and manslaughter, among others.

The study noted the average number of people jailed for life each year since 2001 has been 21, while the average number released each year was four.

There are 61 former life prisoners living in the community, it added.