Chocolate cake with mint chocolate icing...using mint extract in dark chocolate. Possibly garnished with a couple of mint leaves in the middle; not as something people would intentionally eat with each slice, but just like a little cluster of two or three in the center of the whole cake, to look green and alert people of the mint flavor.
Last year, for a St. Patrick's Day cooking class, someone taught dd to make some sort of traditional Irish "potato candy".
They were balls of potato starch and powdered sugar...and maybe coconut?
eta:
Here's a link. I stand corrected: there is no actual potato in the recipe. They're just shaped like potatoes, and rolled in cocoa power for dirt.
http://allrecipes.com/Recipe/Irish-Potato-Candy/ recipe
http://www.bing.com/images/search?q=...C3&FORM=IQFRBA --some more appealing photos, and lol, if you scroll halfway down there really is one recipe that contains mashed potatoes as the starch (it's pictured as a photo of an old newspaper clipping)