You don't tune a bodhran to a key. When people talk about 'a tuneable bodhran' they are referring to one which has mechanisms for adjusting the tension of the skin to get a good tone. Bodhran skins are rawhide, and they lose and absorb water from the atmosphere.

When you pick up a bodhran, listen to the sound of it. If it tintinabulates, producing high-pitched sounds like a cymbal, then the skin is too dry and tight. Rub in a little water, being sure to dampen every millimetre of the surface. (Dry spots will stay tight and might tear.)

If the skin is too damp, it becomes loose and floppy, and the tone when you hit it is a dull thud. I have yet to hear of a really good way to improve this situation. Perhaps a hair dryer, applied gently and evenly from arm's length. Prevention is probably the best thing.

The ideal sound is a rich, warm boom. Listen to talented players till you hear that sound and aim for it.