On 9 November 1973 all I.R.A. prisoners from Mountjoy prison and the Curragh military detention centre were moved to Portlaoise jail. The prisoners were moved in C.I.E. buses with a strong Garda and Army escort. Prisoners from Portlaoise were moved to other prisons to make room for the Republicans.

Three high profile I.R.A. prisoners, J.B. O'Hagan, Seamus Twomey and Kevin Mallon, had spectacularly escaped from Mountjoy by helicopter on 31 October 1973. The move to Portlaoise was made as a measure to contain I.R.A. activity and segregate I.R.A. members from other prisoners.

This footage from RTÉ News shows Gardaí and soldiers outside Mountjoy Prison as IRA prisoners leave by bus for Portlaoise.

Mountjoy Jail, Dublin (1973)