The gun was discovered on waste ground near Carew Park in Southill yesterday afternoon.
A Garda source described the recovery of the gun as “particularly significant”.
Gardaí at Roxboro Road and Henry Street are leading the investigation, which was launched at dawn yesterday.
A senior Garda said the investigation is “ongoing” and will continue into the weekend.
A series of planned Garda raids began at 6.30am and involved gardaí searching homes, premises, and open areas. Five men, including two brothers, were arrested. The men are aged in their late 20s and early 30s.
Garda chiefs set up the planned raids in response to an escalation in violent clashes between members of rival Traveller families.
Superintendent Dan Keane, Roxboro Road, and Superintendent Frank O’Brien of Henry Street, said the arrests were “gang related” and are linked to investigations into ongoing criminality and feuding.
Earlier this week, gardaí were alerted to a morning mass-brawl outside the Social Welfare offices on Dominick Street.
In another tit for tat incident, a man was treated at the Mid-Western Regional Hospital for slash wounds to his face. Windows of a house were also reportedly shot at last weekend.
The five arrested men were being held under Section Four of the Criminal Justice Act. They have to be either charged or released this morning.