The current version is made with reduced-fat vanilla soft serve, whipped light cream, and "Shamrock Shake syrup," according to McDonald's. The soft serve and whipped cream are both made with lots of milk, cream, and sugar, and some preservatives. The flavored syrup is chock-full of high-fructose corn syrup, and, unsurprisingly, various dyes and flavors to make it extra minty and green. The shake does not contain actual shamrocks.
A small Shamrock Shake has 460 calories, a medium has 560 calories, and a large shake has 800 calories. Obviously, these numbers might change if you decide to skip the whipped cream — but there aren't many other adjustments you can make. The vanilla soft serve and shamrock syrup are premade, so you can't really ask to swap the whole milk out for soy or almond, for example.
McDonald's tagline for this frothy green goodness is "cool, minty & full of luck." By that criteria, the luckiest ingredient of them all? Sugar! While using reduced-fat vanilla soft serve and the "light" whipped cream may cut the amount of fat, it does not decrease the jaw-dropping amount of sugar in these shakes.