Shamrock-shaped is an apt descriptor for the decoration of the day, but it is also a useful descriptor of a multi-lobed configuration. For example, in U.S. Patent No. 7,784,624, it was an apt descriptor for the shape of opening of the slot 35 which was designed to hold three bats:

It was also used to describe the cross section of the front part 15 of a bone nail in U.S. Patent No. 9,375,241:

U.S. Patent No. 10,016,703 features a three-leaf shamrock shape in a cartridge enlargement unit:

U.S. Patent No. 6,595,665 has a shamrock-shaped opening:

And there’s the shamrock-shaped of body of a musical instrument, as seen in U.S. Patent No. 3,188,902:

Analogizing a shape of a component to a commonly known object, such as a shamrock, is a convenient and concise way of describing an embodiment of an invention.