You’re allowed to drink alcohol on the cross border enterprise train if you buy it on board. But the conductors will turn a blind eye if you bring your own in a small glass/can if it’s an event such as heading to a concert or football match (my friend is a conductor). But they’re very very strict on a normal NI railways train, you’re not allowed to bring alcohol on board at all. I witnessed several young people have alcohol confiscated on board last year and when we docked in Belfast the police were waiting.
My point is if you’re gonna sneak a drink then sneak a drink, but bringing a whole bottle of Prosecco, glasses and plastering it over public social media for all her followers to see (including young followers) is just plain stupid! It encourages anti-social behaviour on trains.
I hope the conductor confiscated it from them, as they themselves can get reported for allowing it to happen on board during their shift.