Hi All,
I agree that root burls (that's the proper term) can make great club heads. In fact, I've got one sitting in a garage for that some-day project.
That said, not every club head is a burl, and I'll tell you how to tell the difference in a second.
Many shillelaghs actually aren't made from a root burl. Rather, the crafter starts with a tree or shrub (traditionally blackthorn, a type of plum). He selects a trunk or major branch that has many straight, smaller shoots coming off of it. He then cuts the trunk into short sections around the branch, so that the straight shoot becomes the handle of the shillelagh, and the trunk becomes the head. He then carves the head into shape.
The way you tell how it's made is to look at the grain on the head. Burls typically have a confused grain. It looks like random damascus. Trunks have the concentric, circular grain. As an example of a head carved from a trunk,
here's a knobkerrie that Louie posted earlier
Anyway, I admire shillelaghs, and perhaps I'll get one someday. My only problem with them is that they're supposed to be less obvious weapons (the farmer's evening walking stick, really). These days, no one carries a walking stick for walking, so I'd be less obvious if I was walking with a golf club than if I was carrying me blackthorn. If less classy.
Best,
F