By opening two other prisons to male inmates sentenced in courts in eastern counties, Mr McDowell hopes to cut the numbers going to Mountjoy by a quarter.
Overcrowding at the Dublin prison was highlighted by the death of 21-year-old Gary Douch in a basement holding cell containing several other prisoners.
Mr Douch had asked to be moved from the prison landing he was on for his own protection.
His badly battered body was found the next morning under a mattress in the cell.
In an attempt to alleviate conditions, the Midlands and Wheatfield prisons will now also act as committal jails for males sentenced to imprisonment in courts in eastern counties, excluding Dublin, from September under directions put in place by the minister.
“The two new directions which I have signed should help ease the pressures which Mountjoy Prison can come under at peak periods arising from a high number of court committals.
“The Irish Prison Service has indicated that male committals to Mountjoy Prison could be reduced by up to a quarter as a result,” Mr McDowell said.
Fine Gael justice spokesman Jim O’Keeffe TD backed the new measures but warned they would only have a limited impact in the short term.
“I welcome the decision to use Portlaoise and Wheatfield as overspill prisons for most parts of Leinster outside the capital. However, it is late in the day for Mr McDowell to suddenly deal with the huge problem of overcrowding in Mountjoy.
“It took the brutal killing of Gary Douch in Mountjoy, and weeks of almost continuous assaults and rising tension, to bring the reality of the problem home to the minister,” he said.
The Prison Officers’ Association (POA) recently made fresh calls on tackling overcrowding, claiming their lives are in danger.
Under new measures, the Midlands Prison will now take males sentenced to imprisonment from the courts in Carlow, Kildare, Kilkenny, Laois, Offaly, and Westmeath, while Wheatfield Prison will take males sentenced to imprisonment at the courts in Louth, Meath, Monaghan, Wexford and Wicklow.