This wreath is super easy and only three main components: a styrofoam circle wreath, white cotton yarn, and green felt. I used a 12" wreath from Joann's and a 4oz skein of sugar n cream from Joann's as well. The green felt was 2/3 yd remnant that I purchased on the cheap...I can always use a little extra green felt for something.
First, using the yarn and a glue gun, I attached one end of the yarn to the wreath. Then, I tightly wound the yarn around the wreath. Then, I attached the end with the glue gun. Easy.
Next, I made the felt rolls by rolling 24" pieces of felt into a tight circle and using the glue gun to secure the ends.

After making enough felt rolls to evenly space them around the wreath, I made a little shamrock by gluing three felt rolls together and adding a little stem. Then, I attached everything with the gun.
Simple. Green. Irish.
In a similar style, I also made this wreath for my front door.