Thanks for the explanation, Bob. I remembered after I posted the question the term gaffer as in head lighting technician so I half-expected the answer would be electrical tape - the black vinyl stuff used to insulate electrical connections. But duct tape it is.

I found this article on making a cheap banjo and it contains a section on using a piece of synthetic drumhead for a banjo skin, sewing on a flesh hoop, and tensioning with wood screws, that might be germain to the current discussion of bodhran making.