Gardaí are determined to nip the potentially lethal practice in the bud, saying that it is a miracle that nobody has been seriously injured or killed.
They believe that three boys aged nine and 10 were involved in damaging the Expressway Service on the northern outskirts of Glanmire shortly after noon on Monday.
Gardaí have spoken to their parents, but cannot take any further action as the children are too young to face prosecution.
Gardaí are concerned that there could be copycat incidents as schools are currently closed for Easter holidays and they are putting out extra patrols to deter this.
Garda sources have indicated there have been reports of similar incidents in recent days at motorway bridges in the Glanmire and Watergrasshill areas, but none as serious as last Monday’s.
The No. 8 Expressway was on route to Dublin and was passing the pedestrian bridge over the M8 just north of Glanmire when it was damaged by rocks, which shattering the window of the side door at the front of the bus. The coach-style intercity bus was more than half-full at the time.
The driver pulled in to the side of the road, and found that the outside of the double glazed window was damaged, with the interior pane still intact.
The driver immediately reported the incident to Bus Éireann headquarters in Capwell and gardaí were contacted. Despite the damage he decided to continue the journey.
Paul Corby, who was a passenger on the bus, said a number of older people onboard were “quite shaken” by the incident.
“One woman across from me was very visibly shook,” he said.
“We were driving along when we heard a very loud bang. Whoever threw it had good aim,” he said.
Martin Walsh, Bus Éireann regional manager, said it was fortunate that such incidents hadn’t been common in Cork.
“The driver made the decision that it was safe to keep the bus in service, and it will be repaired straight away,” he said.
Gardaí said that motorists who spot people throwing objects from bridges should immediately contact the nearest Garda station.