Two prison units are being opened and 100 cells are being 'doubled up' in a bid to deal with a resurgence in overcrowding in prisons.
The Irish Prison Service said it was also working with the Department of Justice to determine what is driving the increase.
Speaking at the Prison Officers' Association annual conference in Sligo, association president Tony Power said prisoners were again sleeping on mattresses - and he warned that overcrowding was the “perfect atmosphere for the bully to thrive” and for violence to spread.
IPS director general Caron McCaffrey said three of the 12 prisons were currently operating over capacity and that they had 400 more prisoners in the system now compared to the end of 2017.
She said they had a strong plan in place to deal with rising numbers, including the reopening, by the end of the year, of the Training Unit in Mountjoy and the Progression Unit, also in Mountjoy, which will provide 150 extra spaces.
She said an audit was identifying cells that were “capable of holding two prisoners that are only occupied by one”, saying they expected that to give an additional 100 spaces.
Ms McCaffrey said they were also looking at using spare capacity in open centres. In addition, the construction of a new wing at Limerick Prison (due to be completed by 2021) will provide an additional 90 spaces, while a new Limerick female prison will provide an additional 43 spaces.
She said factors for the rise included: an increase in garda numbers which is resulting in more prosecutions; new legislation which is creating new offences and an increase in the general population.
Ms McCaffrey said they were working with the Department of Justice “to understand what's behind the rise” and to put a plan in place.
She said they wanted to implement “impact assessments” on prisons from the effects of new legislation, new policies and funding decisions in other areas of the criminal justice system.
The increase in the prison population is in part due to a sharp rise in remand inmates (those held in custody awaiting trial) from 475 in January 2016 to 714 in January 2019.
In addition, there has been a significant drop in prisoners given temporary release, from 394 to 171 over the same period.
In his address to Justice Minister Charlie Flanagan, POA President Tony Power said “overcrowding has made an unwelcome return to our prisons” with figures rising from 3,745 in April 2017 to 4,049 in April 2019.
Mr Flanagan said he accepted that there was a “challenge of overcrowding”, but said he considered it a “short term” issue.
He said that with a number of initiatives there would be 250 extra spaces by the end of the year, which would be followed by the works at Limerick Prison.