Drinking in the street (or on that green) is illegal due to a Dublin City Council bylaw. There is an exception in place for alcohol consumed outside the pub or restaurant it was purchased in.
You could either argue that he was right and you were breaking the law, while those who bought pints in the pub were not, or that the green does not count as outside the pub, in which case everyone drinking there was breaking the law.
Either way, I couldn't see the Guards being too impressed at him calling them in to hassle well behaved people who just happened to buy their beer from the wrong business.