If you are diriving on rural (or semi rural roads) I think the "de facto" couresy is to flash oncoming motorists to alert them about hazards ahead. I flash for ice, trees/branches, silage contractors, lifestock, dogs, deer, phesants, floods, power lines sparking, vunerable pedestrians, charity collectors, refuse trucks, building site exits (heavy traffic exiting),etc. I ususllay flash the first four or five cars.
In relation to the Go-Safe vans my understanding is that the authorities want to highlight their presence (signs and bright livery on vans). I have notice a marked improvement in driving standards on stretches of road where they operate. Accordingly I flash for the presence of a Go Safe van.
I dont flash for Gardai check points or Customs check points.