Gardai have launched an investigation after a pellet gun was thrown over the wall of Wheatfield Prison on Saturday, the Irish Mirror has reported.
The air gun was spotted by prison officers who quickly retrieved the mock weapon and alerted the Gardai.
It is feared that the imitation gun may have been intended for use in a potential escape bid.
The prison near Clondalkin, Dublin is home to several high-profile criminals including some who are linked the Kinahan drug cartel.