Percussion instruments vary a lot in the way they contribute to the overall pattern. Some are usually played precisely enough to act as timekeepers - snare drum, cajon, triangle, Middle Eastern drums of all kinds, washboard the way I do it. Others provide fills which never seem to land on a primary beat and can add to the overall texture but don't do a heck of a lot to help keep everybody else together - bones work best for this, spoons or shakers are okay in the right hands. The bodhran (or the washboard in the skiffle style) seems almost completely pointless - it never accents the beat in the way any other player can follow and simply provides a continuous stream of thumps of mainly symbolic significance ("hey, we've got a drum, we must be groovy").

It's handy that some of these instruments have different names depending on how they're played. A bendir and a bodhran are the same thing physically, the difference is that the bendir player is a musician.