The number of people jailed for non-payment of of court-ordered fines almost doubled in 2009, according to Irish Prison Service Annual Report.
4,806 people were jailed in 2009, an increase of 91% on the 2008 figure of 2,250. In 2008, there had previously been a jump of 88.7% from 2007.
In total, the number of people sent to prison increased by 13% last year, from 10,928 in 2009 to 12,339 last year.
Of these 88.2% were male, and 11.8% were female.
The Irish Penal Reform Trust (IPRT) has expressed grave concern over further dramatic increases in prison numbers.
It says that not only is the prison population increasing, but the rate of increase is accelerating.