My bodhran is a cheap non-tunable bought from Hobgoblin about 25 years ago.
I apply Neats foot oil once a year to both sides of the skin, leave overnight and wipe off the excess. Don't expect to play it for another day as the skin slowly re-tightens.
For decoration I have used Windor and Newton's inks applied with a small paintbrush rather than a pen. This was applied before any oil!