Shamrock Shake is seasonal, for St. Paddy's day (funny, how come no McCorned Beef & Cabbage Sandwich?).
I recall having one, maybe about 35 years ago, when I was still a kid and liked disgusting sweet crap. It was mint flavored, sweet, and even I thought it was disgusting.
Would have preferred a corned beef shake. Haven't had one since.
Funny, had a McRib, in it's first go around to my knowledge, guessing about 20 years ago. Haven't had one of those since, either.
McDs has a web site claiming McRib is gone for good, but then offering a petition to revive it. Gee, that wouldn't be a marketing ploy would it?