If this were my project, I would haul out my micro cutters (a set of shapes that are more commonly used for polymerclay work) and use the club (as in cards, not as in cavemen) cutter to create the shamrock. A larger shamrock could be created using the tiny heart cutter as a mold for each leaf. As long as you're careful about the quantity of fiber that you use and flat felt it very well - using these tiny cutters/molds works very well. I've done that many times.
You can flat felt yourself some felt cloth and then cut out the shape - just be certain that you really (super really) flat felt your homemade felt cloth really well so that once cut out it doesn't start to fall apart.
I use silk for fur and root it a few strands at a time. It isn't particularly difficult, but it takes a lot of time to cover a complete creature and sometimes (as with this cat) portions of the sculpture have to be furred before being attached- like the lower front legs. It would have been next to impossible to fur those once they were attached to the body.
Learning to control firmness is so useful! There are times when you want something very firm and other times when you don't and almost all the time you need to have a little bit of "give" to allow for making adjustments.
I've made pumpkins over styrofoam that worked quite well (best way to remove the styrofoam is to cut it up - close celled styrofoam won't stick to the fiber as much as open celled styrofoam, but is harder to cut up to remove). I've also make eggs forms over a wooden egg - those were wet felted before I cut it open to remove the egg.