For St Patrick's Day decorations, I put green or white candles in all the candle holders.
I have several leprechaun statues bought at thrift stores, and 6 little black plastic "kettles" that were originally intended for Halloween, bought years ago on clearance for a dime. I crumple a small amount of paper to fill the kettles half way full, then top them off with gold foil chocolate coins. They make wonderful "leprechaun pots of gold", and sit on the bookshelves in the library, in front of the books.
A few years ago, JoAnn's had crocheted shamrocks in white, and in green that were openwork & "puffed out" with a stiffener. Those would not be hard to duplicate & "starch" with a small balloon inside to provide the "poof". Whent he stiffener is dry, pop the balloon & remove.
I have used the decorated sides of green kleenex boxes to cut out shamrocks, to tape on doors, or mirrors.
Last year I made a wreath for the door, using green, white, & gold plastic ornament balls, glued to a circle I cut from styrofoam. I filled in "gaps" with pieces of a green garland bought on clearance ( they sell shamrock garlands at the dollar store that would also work), and filled in with a few green beads & green plastic shamrocks. It turned out really cute, & cost almost nothing.