Look for earrings that have a dangly part that you like and which includes a clear hole through which a hook goes. These are ones I found today for a quarter:
You'll also need hooks. I buy mine at Michaels craft store (they seem to almost always have a 40% off coupon on their website, so be sure to print one!) and it comes out to about $1/pair at my store after coupon. These are the ones I buy:
There are a variety of types and metals available, so pay close attention if you have sensitives (for example, I have better luck with gold or silver plating than with stainless steel). I like to keep both silver and gold on hand so I am ready anytime I find a great pair at a garage sale. My favorite type is French hooks, because if a baby tugs on them, the earring will come out of the hole rather than ripping through your ear (which happened to someone I know).
Take the charm or dangly part of the earring off of its hook using a pair of pliers. This is the pair I like to use.
Most earrings are probably pretty clean, but I like to sanitize them before working with them just in case. For gold, I use 1 drop of dawn dish soap and a splash of ammonia and some hot water in a glass measuring cup. Use a toothbrush if there is visible gunk, but if they look okay, don't scrub with anything. Rinse well after a few minutes, and buff with a clean cotton cloth for optimal shine. For things like beads and painted steel (which is what I suspect these shamrocks are) I use dawn dish soap and hot water, but skip the ammonia for fear it will strip the paint.
Slip the charm onto the hook from the back (poky side).
Voila! Ready to wear! No one will know you only spent $2 and a little elbow grease!