It could be that whomever had it before had used dubbin or similar and had put it on the inside of the drum? (If you do insist on putting it on the drum always on the outside!). Usually you have enough oil on your hands to keep the skin on the bodhran good anyway.

I'd try to get it off with a dry cloth first (or paper towel) and then if that doesn't work then
Try and use a damp cloth (not soaking) but remember if you put anything damp on a bodhran the skin will go very limp and will need to dry out before you play it. (Don't use artificial heat to dry it - just leave it to dry off naturally). I wouldn't recommend using soaps. Maybe (perhaps) a very light amount of vinegar mixed in to the water to lift off some of the dirt. But, make sure it's not too strong.

Regards

Ella who is sooze who is not responsible if things go wrong - in other words try all at your own risk!