Rainbow: Roll each color of fondant or gumpaste into long, narrow log. Place all colors in a tight row and roll over with rolling pin to press them together. Lay flat and twist into curve. Allow to dry until completely hardened (may be a couple days).
Cake: Cover both cakes with green fondant. Roll out light green fondant and cut 1 1/4" strip to fit around larger tier. Emboss with clover leaf. Attach to base of cake by brushing water on cake putting ribbon in place. Cut two thin strips of light green fondant and put around base of smaller tier. Roll light green fondant very thin and cut out letters and two shamrocks. Attach to side of cake using a small amount of water if necessary. Pipe large mounds of buttercream on top of both cakes using an open coupler. Insert dowel into each cake, sticking out about 3-4" to support rainbow. Place rainbow into cloud of buttercream, leaning up against dowel support.
Hand mold leprechaun and pot of gold. Toss confetti sprinkles with gold luster dust to create gold coins to fill pot of gold.