End of the Rainbow
The description of this mission is "Hearts, stars and horseshoes..." which is the popular tag line for Lucky Charms which finishes "Clovers and Blue Moons, pots of gold and rainbows, and the red balloons."
The mission itself has you "Spot the Leprechaun" and then following him to the "Pot of gold" (money stash) at the end of the Rainbow (the cellar).