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It is understood the boys found the gun concealed somewhere in the back garden of a derelict house on the estate.
The weapon was seized by gardai at 7.30pm and the three children have all given detailed statements to officers about what happened.
It is not clear if the boys fired the weapon deliberately or if it went off accidentally while they were playing with it.
Senior sources have revealed that the weapon was stolen from a house in Tyrrellspass, Co Westmeath, in 2007 during the course of a burglary.
A major investigation has now begun with officers trying to establish if the gun had been used in any other crimes.
"I couldn't believe what I was hearing when Matthew ran over to me and told me what happened," said Laura, who lives across the road from the abandoned house where the shocking incident occurred.
"He said 'mammy, we're after finding a gun'. I have told him time and time again not to be playing in the houses and that it is dangerous, but boys will be boys," she added.
"Matthew said he had been building a fort with a friend when they discovered the gun, and one of them pulled the trigger," said Laura.
"Matthew was shaking when he came home, I was shaking for hours afterwards. Any one of them could have been killed," she added.
"There are too many abandoned houses here, and it is dangerous. I don't know who needs to do something about them, the owners or the council," Laura explained.
"When I saw them coming out with the gun. I nearly died with fright," she added.