Genuine Irish man here.
The first slogan that pops to mind when you mention green beer is
"Stout....because green beer is for American tourists", but then I realised I'm insulting the wrong demographic, but it'd be appreciated over here for what it is
"Stout....because green beer is for Amadáns." would be an alright one, where Amadán is the Irish word for fool, it's a polite enough insult too, your granny might call you an Amadán for example, but you'd not hear it in a pub.
"stout.....because green beer is for Bollixes" would be kinda Dublincentric.