The guard has been charged with two counts of attempted murder and two of assault with a deadly weapon.
Justin Donaghy and Gerard Hourigan, both 29, remain in hospital after being seriously injured in the shooting outside a bowling alley in Queens in New York more than a week ago.
Michael Iavecchio, 54, a former prison guard, was first charged with a less serious second degree assault.
Iavecchio is believed to have told police he was attacked by at least one of the two Irishmen, whom he claimed to have escorted out of the premises because they were smoking.
But after reviewing footage from cameras at the AMF bowling alley and talking to witnesses, police and prosecutors decided to dramatically increase the seriousness of the charges.
It means prosecutors believe he was not provoked or acting in self defence when he pulled his .380 Rutger handgun in the early hours of Saturday, February 6.
And he shot to kill, prosecutors will argue.
Iavecchio, who is out on $25,000 cash bail, has been issued with two orders to not go near the two men and ordered to hand over all his weapons. He is due in court again on Friday.
As a former prison captain Iavecchio had a permit to carry a concealed weapon.
Mr Donaghy’s wife, Bonnie, witnessed the shooting as she left the bowling alley after the two men.
Mr Donaghy, from Navan, Co Meath, was shot in his right side. He could be released from hospital within the next few days.
Limerick native Mr Hourigan, was shot in the stomach. He is in a more serious condition, their lawyer, JP Delaney, said.
“But both are in good form. They are awake and talking,” said Mr Delaney.
The two men have been charged with menacing and harassment. They have been told to appear in court at the end of March. “They are minor charges. If they had just been arrested and charged with these offences, they would have gone to the precinct and been out in an hour,” the lawyer said.