A more long term solution involves the use of cardboard 'shims'. These can be placed inside the bodhran, between the top of the rim and the skin, thereby increasing the tension of the skin. Long, thin, curved strips of corrugated cardboard work well. Add as many as you need in order to remove any slack in the skin. The steamy conditions should make the skin slacken enough that the shims can be removed.
Dubbin is available in shoe stores and ski supply shops.
It is a little more difficult to permanently tune down a taut drum than to tune up a slack one. Saturating a tight skin with oils or moisturizer will loosen the skin and decrease the tone by a few notes. The effect of this may last from a few days to a few months. This serves to immediately and drastically reduce the pitch of the drum. Depending on how much water is applied, you can tune the bodhran down to its lowest playable note.