the mcd's on 6th ave b/w 46 and 47th have them...limited time thing.
never tried it myself...actually never heard of it until last year around st. patrick's day a friend of mine mentioned he was seraching for them...i checked and this location, across from my office, had them. here we are again, and here they are again.
I found my white whale(or green as the case may be). Shamrock Shakes at the Mc Donalds in Union Square. Even though there are signs on the doors and behind the register person's head they still acted like I was speaking in tongues when I requested the shakes. Did it live up to my memory? Well not in flavor, but at least the color was the appropriate minty green. I guess they were never all that good but I've always loved green foods. Now, another 12 months to rest and forget what they taste like until I crave them again.
Thanks for your leads,