It’s hard to determine the true origins of the shillelagh. How can you pinpoint the first time someone picked up a stout stick and brained someone else with it? And the shillelagh really hasn’t evolved too much beyond that. Some prize blackthorn wood, while some might fill the head with molten lead. Others might bind it with leather straps, or cure it with animal fat. But, at the end of the day, it really is just a bloody heavy stick that cracks skulls.
The Shillelagh possesses very high knockdown at the cost of low damage. If damage is not the focus, it can be seen as a quicker version of the Ding Dong Breaching Tool as both possess high knockdown. However, its low performance overall makes it outclassed by other melee weapons, most noticeably by the free Lucille Baseball Bat.