Hardwood is often used because of its weight and durability. I've seen maple, walnut and oak beaters. Softwood will work although it's lighter and will dent slightly if you like to tap the rim while playing. Most beaters are turned on a lathe, but you can hand carve one that will work just as well. Beaters are commonly six to eight inches long, but it's O.K. to experiment with length.
Thickness can also vary quit a bit.
If you're not fussy about looks, a piece of hardwood broomstick with the ends rounded and grips cut into it works great. The most important thing to keep in mind when making a beater is to sand away any abrasive or sharp areas which could puncture or scratch the drum skin when playing. Some bodhran players forgo the use of a beater entirely and just hit the drum with the knuckles of their hand.