The vast majority of these incidences (which are still fairly rare) happen outside the ground.
Security outside of grounds is the responsibility of the Gardai.
In many other countries, the police force assess the level of security required in and around the ground and the club are required to pay directly for the policing. This is not the case in Ireland.
If this was the case, clubs would be unable to afford the cost of adequate policing.
Catch 22.