A number of Bus Eireann drivers are on sick leave following assaults by passengers.
There were seven assaults on drivers in the last couple of months alone.
The National Bus and Rail Union is calling for safety screens to be retro-fitted to the entire bus fleet.
Union Representative Stephen Nugent said drivers can also be easily robbed.
Mr Nugent said: "Only a small percentage of our fleet has safety screens, whereas if you look at the Dublin Bus model, all their buses have safety screens.
"We are wide open and the difference between ourselves and other buses is that we have a very substantial amount of cash, unlike Dublin Bus where the coin goes into a safety box and it's impossible to get to. We are totally exposed."