Mama Llama,
I think a lot of the "blame" for this can be put on Pinterest. I think that parents see that other parents may be celebrating holidays in a certain way and feel like they have to match that or up the ante. It feels like this is a bit of a recent development. The last few years everyone is suddenly doing an Elf-on-the-Shelf tradition at Christmas and all sorts of other extras.
Anyway. I think that you just need to do what YOU want to do. Our family DOES celebrate St. Patrick's Day but that is simply because I personally find that kind of thing fun. I will tell you that starting a celebration does set up an expectation that it will continue every year, so that is hard.
Perhaps you can go simple and just spend a few minutes tonight talking about who St. Patrick is and what St. Patrick's Day is about. You can explain that generally, most people just wear green and in some places there is a parade. Leprechauns do not leave gold- the legends say they are notorious for protecting their gold stash at all costs!
If you want to do more than that, there are certainly lots of fun ideas online, but I would suggest that 1- you only do what you feel comfortable with and keep in mind kids do start to expect it every year , and 2-don't just do it because her classmates have that tradition.
When we were kids, my mom simply packed us a green lunch (green jello, green drink, green rice krispie square, cucumber slices, possibly a green sandwich - she did find green bread one year, it was horrendous looking, ha ha! ) Our family does do a leprechaun but he writes notes or jokes, plays tricks, and we set traps to try to catch him all day, and we do green breakfast (lots of food coloring involved) and a traditional Irish dinner. And again, that is just because it is fun for ME and the kids, otherwise I would not be doing it.