What the heck is a Shamrock shake, anyhow? Since actual shamrock has no culinary uses...
It's just a mint-flavored milkshake, dyed green for Saint Patrick's day. It's nothing special.
Buuuuuuut, when combined with a
chocolate milkshake, it creates the mythical Chocolate Shamrock shake...a delightful blend of minty chocolate frozen goodness - a completely separate animal from just a plain Shamrock shake. It's important to note that what makes it good is mint - NOT peppermint. So the menthol levels are reduced from say a peppermint candy cane down to a Girl Scout Thin Mint cookies level. Kind of like how mint & chip ice cream doesn't taste like a Junior Mint or a Peppermint Patty. Actually, green mint & chip ice cream is a good comparison...it's basically a milkshake version of that, but with more tasty chocolate in it to balance it out.
For a McDonald's milkshake, a chocolate-mint version is most excellent.