I actually made these cupcakes for my sister's birthday, but they work for St. Patrick's Day as well. Super easy to assemble, the only trick was getting the rainbows to stand up the way I wanted them too - especially in the often humid Texas weather. I tried both frosting (not stable enough) and marshmallows (didn't look enough like clouds) before I hit on the answer. Below is a step-by-step pictorial.
I can't claim the original idea for these, as I have seen several rainbow cupcakes out there on Pinterest. They even make kits for these now (although I don't recommend them - the hard gumpaste clouds are unappetizing and the rainbow strips, at least in the kit I saw, are dull and not as colorful as these).
I used funfetti cake mix and cream cheese icing for the cupcakes. Rainbow gummi strip candy for the rainbows, and mini-marshmallows and spray-can white frosting to make the clouds.
Step 1 - Ice your cupcakes sky-blue
Step 2 - Cut rainbow strips. I found that about a quarter-inch shorter than the long side of a credit card is the optimal length.
Step 3 - Place your mini-marshmallows about where you want either side of the rainbows to hit, about a quarter to a half-inch away from the outer edges of the cupcake. You want them to be on situated far enough away from the edge so that they are fairly level.
Step 4 - Place your rainbow. Adjust your marshmallows to make a perfect arch.
Step 5 - Pipe icing on top of around and on top of marshmallows to create the clouds.