Miranda. about the Shamrock cake pan. Why not make your own design. Make 2 round layers, then use a paper pattern and cut them to the shape you want. When my kids were growing up, there were not shaped cake pans, and if there were, I could not afford to buy them. And why buy one for using once a year?
I made all the Birthday cakes as cut-up cakes. I even made Opus the penquin for my daughter. You do have to be sure to make the cuts deep enough so that when you apply the frosting, it will still look like a Shamrock. You will be fortunate enough to be able to make green frosting, BUT in those days Black was not an option and I don't recall now how I did it, but it passed inspection. She was delighted. WE have pictures somewhere around here or there (at her house.) You can do this.
Enjoy your own creativity.