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:

U.S. Patent No. 7,784,624

It was also used to describe the shape of the catheter passageway in U.S. Patent No. 7,207,981; the cross section of the front part 15 of a bone nail in U.S. Patent No. 9,375,241:

U.S. Patent No. 9,375,241

or the three-leaf shamrock shape of the cartridge enlargement unit 610 in U.S. Patent No. 10,016,703:

U.S. Patent No. 10,016,703

or the shamrock-shaped opening in U.S. Patent No. 6,595,665:

U.S. Patent No. 6,595,665

or the shamrock-shaped of body 6 of the musical instrument in U.S. Patent No. 3,188,902:

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.