There's always a lot of debate about what is, and what is not, the true shamrock. The stylised version looks a lot like an Oxalis and I've heard people claim the shamrock is an Oxalis. But when I visited my cousin in County Cork years ago, the shamrock she showed me was Trifolium dubium, a little yellow clover. This, she told me, is the plant folks wear on St Patrick's Day in Rosscarbery, Skibbereen, and Cork.
Shamrock, Trifolium dubium
Sorry about my scrappy photo. Wikipedia, on the other hand, tells us the shamrock is the white clover, Trifolium repens:
Whichever species is the true shamrock, it's a three-leaved (leafletted) clover, not a four-leaved. The luck of the Irish doesn't need four-leaved clovers, it seems. Nothing demonstrates that better than tonight's result (Ireland 15, Australia 6).