This question comes up once in a while and usually the guy who posts it gets picked on a bit. I personally do not plan a green beer, but you could add some green food coloring to any lighter style like a wheat beer or pale ale.
You could try using something like Spirulina or, as has been suggested, green food coloring. The food coloring is so strong that it overpowers the yellow/amber color of the beer easily, so you don't need to use blue.
All snarking aside, there isn't any way to make a green-colored beer except by adding food coloring. Irish brewing is more towards the dark side and even the lagers tend to be somewhat red. You might look into Cream Ale recipes.