There Could Be Fish Parts In Your St. Patrick's Day Guinness
The fish comes from a gelatin-like substance called Isinglass. Its made from the air-bladders of fish like sturgeon and is added to cask beers. It's purpose is to grab any solid particles and yeast cells that are in the beer into a jelly-like mass, which settles to the bottom of the cask and can then be removed.
Theoretically, that removal means all the isinglass is taken out of the beer, but technically, Guinness admits, there could still be some in the beer. Smithsonian Blogs has a whole history of isinglass if you want to know more.
Surprisingly, many wines are also made with gelatin products, so they aren't vegan either.
Hoping for a mixed drink? Some of those are out too: Sodas are sometimes made with ester gum, which is made of glycerol, which sometimes comes from animals. Orange juices that contain added omega-3 fatty acids are also usually non-vegan, since the fats are usually from fish oil. Ocean Spray Ruby Red Grapefruit juice is dyed pink using a dye derived from crushed insects called carmine, similar to the dye that had been used in Starbucks's Strawberry Frappuccino that caused a kerfuffle in 2012.
The use of isinglass has decreased with modern brewing methods. Not all brands of beer use fish glue to clarify their product. Here's a database of beers and if they are vegan, to prep your beverage options for St. Patrick's partying.