Myself, Cara and a few others were there about 2 year ago but had a pretty bad experience with the staff. They didn't sell craft but there was a pile of people just drinking carryouts on the green outside and enjoying the sunshine so we just went and got craft from Sweeneys. Then a staff member from the Gravediggers came out and tried to tell us we could only drink on the green if we bought drink from their pub. Cara asked him if they owned the green or were had public liability insurance for it but he then got stroppy and threatened to call the guards. They never challenged any of the other groups either. I think your man heard the northern accents and thought he could act the big man. It left a sour note for me and I'll not be back.
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.
Is there a limit to how far away from the pub you can be though? I assumed that law was more to cover pubs/restaurants who have on street seating and the like. The Gardai did do a loop round at one stage but didn't stop to move anyone on anyway.
Just read the [url=http://www.dublincity.ie/main-menu-services-recreation-culture/intoxicating-liquor-bye-laws:1j4u2jcx]bylaws[/url:1j4u2jcx] again and you are absolutely right. The exemption only applies to people seated outside the pub in seating provided by the pub for their patrons exclusive use. Everyone drinking on that green was breaking the law.
The Gardaí rightly took a look and decided not bother people who were doing no harm.