Well, it's not green, and not really Irish, but it's got Guinness in it so it counts in my book!
Fill a champagne flute (or similar tall, thin class) halfway with champagne/sparkling wine. Once it settles, carefully top with cold Guinness, using a bar spoon to prevent the two from mixing. The Guinness should float on top of the champagne.
According to Wikipedia, black velvets were first created in 1861 to mourn the death of Prince Albert, symbolizing the black armbands worn by mourners. It really has nothing to do with St Patrick's Day, but I'm going to drink it anyway.