Depends on whether your talking about the Cocktail or the flourescent Green pints you often see people downing which are also popularly know as "green monsters"
if it's the pints then they are nothing more than a Snakebite (lager and Cider) with a shot or two of Blue Curacao in it .
The most popular Blue Curacao liquer is made by Bols.
The Blue liquer turns Bright flourescent green when it's mixed with the snakebite.
Another variation is served by a local pub of mine under the name "Castrol GTX". By replacing the lager with Scotch you get something that resembles the aforementioned oil.
Drink, but very carefully.....